Code reference documentation update

[ajstewart]

Added a code reference section to the documentation

• New docs dependency of mkdocs-gen-files.
• Auto generates pages using mkdocs-gen-files.
• Updated non-google format docstrings to google format.

Note that this is based on #471.

[ajstewart]

@marxide @srggrs (cc @dliptai) I've gone through and activated the code reference using mkdocstrings, so we have begin to have some code referencing in the documentation. Most of the work involved updating all the docstrings to the google format and I've also filled in a lot of empty docstrings.

Again it's probably much easier to serve the docs and look at them directly.

I know there is some majorish work going on with some of the functions which will make the docstrings obsolete but maybe this seeing as this is really only changing the docstrings I'm hoping it will be quite smooth to maybe merge this first and then rebase other branches?

Some notes:

• This is based on my other branch in Documentation update #471 - to avoid that one getting huge.
• I've activated it only on the vast_pipeline directory.
• views.py and serializers.py are problematic (see the comment in the code).
• I have left out the migrations and tests sub-directories.
  • I left out tests simply because there is a lot in there and I wasn't sure I had the time. But it should be updated and gone through by someone.
• I'm using mkdocs-gen-files to automatically generate the markdown files, this is done in the script docs/gen_doc_stubs.py, which is also where I define which files and directories to skip.
• I follow the PEP 484 type annotations format in the google docstrings.
• I'm unsure on the debug function in auth.py there is a TODO here.
• The main module comments at the top may be inconsistent - i.e. I might have missed some, please feel free to just add these in if you see one.