docs: [FC-0074] add ADR with event design suggestions

[mariajgrimaldi]

Description

This PR adds an ADR with suggestions on how to design a new event based on the Building Event-Driven Microservices 3rd chapter about Communication and Data Contracts and Martin Fowler's Event Driven article. I'm proposing that these practices be considered when implementing new events, but they should NOT be considered standards that all events should strictly follow. Also, I'm not saying either that all the existing events in the library comply (or should) with these practices.

This document will be used as a reference in the How to Create a New Event guide. I'm currently validating these items while I write the guide, making sure these suggestions are clear enough to follow. You can review it here: #439

Testing instructions

Review here: https://docsopenedxorg--438.org.readthedocs.build/projects/openedx-events/en/438/decisions/0016-event-design-practices.html

Deadline

None

Checklists

Check off if complete or not applicable:

Merge Checklist:

• All reviewers approved
• Reviewer tested the code following the testing instructions
• CI build is green
• Version bumped
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• Code dependencies reviewed
• Fixup commits are squashed away
• Unit tests added/updated
• Noted any: Concerns, dependencies, migration issues, deadlines, tickets

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• Create a tag
• Create a release on GitHub
• Check new version is pushed to PyPI after tag-triggered build is
  finished.
• Delete working branch (if not needed anymore)
• Upgrade the package in the Open edX platform requirements (if applicable)

[openedx-webhooks]

Thanks for the pull request, @mariajgrimaldi!

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[mariajgrimaldi]

@robrap @bmtcril I'd like to know what you folks think about this proposal based on your experience working with this architecture.

[mariajgrimaldi]

FYI @Squirrel18 @Alec4r