Update backward extremity docs to make it clear that it does not indicate whether we have fetched an events' `prev_events` (#11469)

Spawning from https://github.com/matrix-org/synapse/pull/9445#discussion_r758958181



Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

* Labeled a lot more code blocks with the appropriate type
* Fixed a couple of minor typos (missing/extraneous commas)

Signed-off-by: Sumner Evans <me@sumnerevans.com>

Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

- Use sytest:bionic. Sytest:latest is two years old (do we want
  CI to push out latest at all?) and comes with Python 3.5, which we
  explictly no longer support. The script now runs under PostgreSQL 10
  as a result.
- Advertise script in the docs
- Move pg testing script to scripts-dev directory
- Write to host as the script's exector, not root

A few changes to make it speedier to re-run the tests:

- Create blank DB in the container, not the script, so we don't have to
  `initdb` each time
- Use a named volume to persist the tox environment, so we don't have to
  fetch and install a bunch of packages from PyPI each time

Co-authored-by: reivilibre <olivier@librepush.net>

The major change is moving the decision of whether to use oEmbed
further up the call-stack. This reverts the _download_url method to
being a "dumb" functionwhich takes a single URL and downloads it
(as it was before #7920).

This also makes more minor refactorings:

* Renames internal variables for clarity.
* Factors out shared code between the HTML and rich oEmbed
  previews.
* Fixes tests to preview an oEmbed image.

Point to the book where possible, and use hyperlinks to github to refer to files not included in the book.

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Add developer documentation to explain room DAG concepts like `outliers` and `state_groups` (#10464)

This is essentially an implementation of the proposal made at https://hackmd.io/@richvdh/BJYXQMQHO, though the details have ended up looking slightly different.