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

Co-authored-by: Sean Quah <8349537+squahtx@users.noreply.github.com>

* Recommend poetry in docs

- readme
- contributor guide
- upgrade notes
- new dev cheat sheet for poetry

Co-authored-by: Shay <hillerys@element.io>
Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Flesh out documentation for running SyTest against Synapse, including use of Postgres and worker mode. (#12271)

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

Co-authored-by: Patrick Cloke <clokep@users.noreply.github.com>

* Docs: add missing PR submission process how-tos

The documentation says that in order to submit a pull request you have to run the linter and links to [Run the linters](https://matrix-org.github.io/synapse/latest/development/contributing_guide.html#run-the-linters).  IMO "Run the linters" should explain that development dependencies are a pre-requisite.

I also included `pip install wheel`  which I had to run inside my virtual environment on ubuntu before I `pip install -e ".[all,dev]"` would succeed.

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>

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>