Move dev/ docs to development/ (#10453)

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-Some notes on how we use git
-============================
-
-On keeping the commit history clean
------------------------------------
-
-In an ideal world, our git commit history would be a linear progression of
-commits each of which contains a single change building on what came
-before. Here, by way of an arbitrary example, is the top of `git log --graph
-b2dba0607`:
-
-<img src="git/clean.png" alt="clean git graph" width="500px">
-
-Note how the commit comment explains clearly what is changing and why. Also
-note the *absence* of merge commits, as well as the absence of commits called
-things like (to pick a few culprits):
-[“pep8”](https://github.com/matrix-org/synapse/commit/84691da6c), [“fix broken
-test”](https://github.com/matrix-org/synapse/commit/474810d9d),
-[“oops”](https://github.com/matrix-org/synapse/commit/c9d72e457),
-[“typo”](https://github.com/matrix-org/synapse/commit/836358823), or [“Who's
-the president?”](https://github.com/matrix-org/synapse/commit/707374d5d).
-
-There are a number of reasons why keeping a clean commit history is a good
-thing:
-
- * From time to time, after a change lands, it turns out to be necessary to
-   revert it, or to backport it to a release branch. Those operations are
-   *much* easier when the change is contained in a single commit.
-
- * Similarly, it's much easier to answer questions like “is the fix for
-   `/publicRooms` on the release branch?” if that change consists of a single
-   commit.
-
- * Likewise: “what has changed on this branch in the last week?” is much
-   clearer without merges and “pep8” commits everywhere.
-
- * Sometimes we need to figure out where a bug got introduced, or some
-   behaviour changed. One way of doing that is with `git bisect`: pick an
-   arbitrary commit between the known good point and the known bad point, and
-   see how the code behaves. However, that strategy fails if the commit you
-   chose is the middle of someone's epic branch in which they broke the world
-   before putting it back together again.
-
-One counterargument is that it is sometimes useful to see how a PR evolved as
-it went through review cycles. This is true, but that information is always
-available via the GitHub UI (or via the little-known [refs/pull
-namespace](https://help.github.com/en/github/collaborating-with-issues-and-pull-requests/checking-out-pull-requests-locally)).
-
-
-Of course, in reality, things are more complicated than that. We have release
-branches as well as `develop` and `master`, and we deliberately merge changes
-between them. Bugs often slip through and have to be fixed later. That's all
-fine: this not a cast-iron rule which must be obeyed, but an ideal to aim
-towards.
-
-Merges, squashes, rebases: wtf?
--------------------------------
-
-Ok, so that's what we'd like to achieve. How do we achieve it?
-
-The TL;DR is: when you come to merge a pull request, you *probably* want to
-“squash and merge”:
-
-![squash and merge](git/squash.png).
-
-(This applies whether you are merging your own PR, or that of another
-contributor.)
-
-“Squash and merge”<sup id="a1">[1](#f1)</sup> takes all of the changes in the
-PR, and bundles them into a single commit. GitHub gives you the opportunity to
-edit the commit message before you confirm, and normally you should do so,
-because the default will be useless (again: `* woops typo` is not a useful
-thing to keep in the historical record).
-
-The main problem with this approach comes when you have a series of pull
-requests which build on top of one another: as soon as you squash-merge the
-first PR, you'll end up with a stack of conflicts to resolve in all of the
-others. In general, it's best to avoid this situation in the first place by
-trying not to have multiple related PRs in flight at the same time. Still,
-sometimes that's not possible and doing a regular merge is the lesser evil.
-
-Another occasion in which a regular merge makes more sense is a PR where you've
-deliberately created a series of commits each of which makes sense in its own
-right. For example: [a PR which gradually propagates a refactoring operation
-through the codebase](https://github.com/matrix-org/synapse/pull/6837), or [a
-PR which is the culmination of several other
-PRs](https://github.com/matrix-org/synapse/pull/5987). In this case the ability
-to figure out when a particular change/bug was introduced could be very useful.
-
-Ultimately: **this is not a hard-and-fast-rule**. If in doubt, ask yourself “do
-each of the commits I am about to merge make sense in their own right”, but
-remember that we're just doing our best to balance “keeping the commit history
-clean” with other factors.
-
-Git branching model
--------------------
-
-A [lot](https://nvie.com/posts/a-successful-git-branching-model/)
-[of](http://scottchacon.com/2011/08/31/github-flow.html)
-[words](https://www.endoflineblog.com/gitflow-considered-harmful) have been
-written in the past about git branching models (no really, [a
-lot](https://martinfowler.com/articles/branching-patterns.html)). I tend to
-think the whole thing is overblown. Fundamentally, it's not that
-complicated. Here's how we do it.
-
-Let's start with a picture:
-
-![branching model](git/branches.jpg)
-
-It looks complicated, but it's really not. There's one basic rule: *anyone* is
-free to merge from *any* more-stable branch to *any* less-stable branch at
-*any* time<sup id="a2">[2](#f2)</sup>. (The principle behind this is that if a
-change is good enough for the more-stable branch, then it's also good enough go
-put in a less-stable branch.)
-
-Meanwhile, merging (or squashing, as per the above) from a less-stable to a
-more-stable branch is a deliberate action in which you want to publish a change
-or a set of changes to (some subset of) the world: for example, this happens
-when a PR is landed, or as part of our release process.
-
-So, what counts as a more- or less-stable branch? A little reflection will show
-that our active branches are ordered thus, from more-stable to less-stable:
-
- * `master` (tracks our last release).
- * `release-vX.Y` (the branch where we prepare the next release)<sup
-   id="a3">[3](#f3)</sup>.
- * PR branches which are targeting the release.
- * `develop` (our "mainline" branch containing our bleeding-edge).
- * regular PR branches.
-
-The corollary is: if you have a bugfix that needs to land in both
-`release-vX.Y` *and* `develop`, then you should base your PR on
-`release-vX.Y`, get it merged there, and then merge from `release-vX.Y` to
-`develop`. (If a fix lands in `develop` and we later need it in a
-release-branch, we can of course cherry-pick it, but landing it in the release
-branch first helps reduce the chance of annoying conflicts.)
-
----
-
-<b id="f1">[1]</b>: “Squash and merge” is GitHub's term for this
-operation. Given that there is no merge involved, I'm not convinced it's the
-most intuitive name. [^](#a1)
-
-<b id="f2">[2]</b>: Well, anyone with commit access.[^](#a2)
-
-<b id="f3">[3]</b>: Very, very occasionally (I think this has happened once in
-the history of Synapse), we've had two releases in flight at once. Obviously,
-`release-v1.2` is more-stable than `release-v1.3`. [^](#a3)