Docs: add missing PR submission process how-tos (#11821)

* 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.

changelog.d/11821.doc

@@ -0,0 +1 @@
+Add missing steps to the contribution submission process in the documentation.  Contributed by @sequentialread.

docs/development/contributing_guide.md

@@ -55,6 +55,7 @@ setup a *virtualenv*, as follows:
 cd path/where/you/have/cloned/the/repository
 python3 -m venv ./env
 source ./env/bin/activate
+pip install wheel
 pip install -e ".[all,dev]"
 pip install tox
 ```
@@ -116,7 +117,7 @@ The linters look at your code and do two things:
 - ensure that your code follows the coding style adopted by the project;
 - catch a number of errors in your code.
 
-They're pretty fast, don't hesitate!
+The linter takes no time at all to run as soon as you've [downloaded the dependencies into your python virtual environment](#4-install-the-dependencies).
 
 ```sh
 source ./env/bin/activate