* 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.