Reorganise the database schema directories (#9932)

The hope here is that by moving all the schema files into synapse/storage/schema, it gets a bit easier for newcomers to navigate.

It certainly got easier for me to write a helpful README. There's more to do on that front, but I'll follow up with other PRs for that.

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+# Synapse Database Schemas
+
+This directory contains the schema files used to build Synapse databases.
+
+Synapse supports splitting its datastore across multiple physical databases (which can
+be useful for large installations), and the schema files are therefore split according
+to the logical database they are apply to.
+
+At the time of writing, the following "logical" databases are supported:
+
+* `state` - used to store Matrix room state (more specifically, `state_groups`,
+  their relationships and contents.)
+* `main` - stores everything else.
+
+Addionally, the `common` directory contains schema files for tables which must be
+present on *all* physical databases.
+
+## Full schema dumps
+
+In the `full_schemas` directories, only the most recently-numbered snapshot is useful
+(`54` at the time of writing). Older snapshots (eg, `16`) are present for historical
+reference only.
+
+## Building full schema dumps
+
+If you want to recreate these schemas, they need to be made from a database that
+has had all background updates run.
+
+To do so, use `scripts-dev/make_full_schema.sh`. This will produce new
+`full.sql.postgres` and `full.sql.sqlite` files.
+
+Ensure postgres is installed, then run:
+
+    ./scripts-dev/make_full_schema.sh -p postgres_username -o output_dir/
+
+NB at the time of writing, this script predates the split into separate `state`/`main`
+databases so will require updates to handle that correctly.