Add sphinx extension

[ewjoachim]

Closes #865

Procrastinate Tasks don't get documented by Sphinx Autodoc. It's a bit sad, but with a very short sphinx extension, we can solve it. So... I guess we have a new contrib.

Successful PR Checklist:

• [x] Tests
  • [ ] (not applicable?)
• [x] Documentation
  • [ ] (not applicable?)

PR label(s):

• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20breaking%20%F0%9F%92%A5
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20feature%20%E2%AD%90%EF%B8%8F
• [x] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20bugfix%20%F0%9F%95%B5%EF%B8%8F
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20miscellaneous%20%F0%9F%91%BE
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20dependencies%20%F0%9F%A4%96
• [ ] https://github.com/procrastinate-org/procrastinate/labels/PR%20type%3A%20documentation%20%F0%9F%93%9A

[github-actions[bot]]

Coverage report

Click to see where and how coverage changed

File	Statements	Missing	Coverage	Coverage (new stmts)	Lines missing
procrastinate/contrib/sphinx
__init__.py
Project Total

This report was generated by python-coverage-comment-action

[medihack]

Nice. Procrastinate is full of little details. Let's get it in there :-)