(#5849) Convert rst to markdown (#6040)

Converting some of the rst documentation to markdown.  Attempted to
preserve whitespace and line breaks to minimize cosmetic change.

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+# Media Repository 
+
+*Synapse implementation-specific details for the media repository*
+
+The media repository is where attachments and avatar photos are stored.
+It stores attachment content and thumbnails for media uploaded by local users.
+It caches attachment content and thumbnails for media uploaded by remote users.
+
+## Storage
+
+Each item of media is assigned a `media_id` when it is uploaded.
+The `media_id` is a randomly chosen, URL safe 24 character string.
+
+Metadata such as the MIME type, upload time and length are stored in the
+sqlite3 database indexed by `media_id`.
+
+Content is stored on the filesystem under a `"local_content"` directory.
+
+Thumbnails are stored under a `"local_thumbnails"` directory.
+
+The item with `media_id` `"aabbccccccccdddddddddddd"` is stored under
+`"local_content/aa/bb/ccccccccdddddddddddd"`. Its thumbnail with width
+`128` and height `96` and type `"image/jpeg"` is stored under
+`"local_thumbnails/aa/bb/ccccccccdddddddddddd/128-96-image-jpeg"`
+
+Remote content is cached under `"remote_content"` directory. Each item of
+remote content is assigned a local `"filesystem_id"` to ensure that the
+directory structure `"remote_content/server_name/aa/bb/ccccccccdddddddddddd"`
+is appropriate. Thumbnails for remote content are stored under
+`"remote_thumbnails/server_name/..."`