feat: Add a config for openedx-events annotations

[bmtcril]

Description: Adding code and config to find and document annotations used in the openedx-events repository. When run in openedx-events (see openedx/openedx-events#318) the generated documentation looks like this:

[timmc-edx]

My review shouldn't be considered authoritative as I'm not a maintainer and I'm not familiar with this repo, but -- I did review the fields/annotations for completeness and consistency.

[bmtcril]

@mariajgrimaldi thanks, the docs said that first argument was supposed to be a separator so I thought it would... do that. :) All set now.

[mariajgrimaldi]

@timmc-edx: this implementation is similar to the featuretoggles one

code-annotations/code_annotations/contrib/sphinx/extensions/featuretoggles.py

Line 26 in 786da8c

class FeatureToggles(SphinxDirective):

with implementation-specific changes to generate events docs instead. I understand you're not the maintainer of the repo, so could we include someone familiar with the code? Because of the similarity with the other extensions, I don't think it'd be difficult to review.

[timmc-edx]

I've attempted to ping the owning team within 2U, but haven't heard back yet. I think if another week goes by without hearing from them, we should just merge this.

[timmc-edx]

@bmtcril I think we're good to merge this.