feat: Add autodoc for event annotations

[bmtcril]

Description: Adds automatic documentation of annotated events for discoverability and ease of configuration. This is in draft until dependencies merge and release, so I can update requirements to the official version, I'll be adding event_key_fields for the rest of the events while that is in review. Generated docs look like this:

Dependencies: openedx/code-annotations#98

Installation instructions: List any non-trivial installation
instructions.

Testing instructions:

1. Requirements are updated to use the branch of code-annotations necessary to run this so pulling this branch and running make requirements should suffice
2. Run make docs
3. Sphinx should run and pop open the generated docs in your browser. Navigate to reference -> events to see the content.

Reviewers:

• tag reviewer
• tag reviewer

Merge checklist:

• All reviewers approved
• CI build is green
• Version bumped
• Changelog record added
• Documentation updated (not only docstrings)
• Commits are squashed

Post merge:

• Create a tag
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)

[bmtcril]

Coverage issue is just because there is more code (comments), there is nothing to test in any of this.

[mariajgrimaldi]

This has my soft-approval. Thanks for doing this! We'll merge this once openedx/code-annotations#98 is merged so we can the code-annotations release instead. Is that okay?

[timmc-edx]

Looks good pending the code-annotations release!

[bmtcril]

@timmc-edx @mariajgrimaldi this is ready for re-review. Changes since the last push:

• Using the release of code-annotations
• Ran make upgrade because of that
• Removed pii checks from Makefile and tox since they were broken after the removal of .pii_annotations.yml

[mariajgrimaldi]

LGTM. Thank you!