Transition to new process for updating changelogs

[mcmire]

Explanation

Currently, we ask that when contributors submit a pull request, they fill out a Changelog section in the description which lists all of the consumer-facing changes they've made. This section is ultimately used when creating a new release to assist with populating CHANGELOG.md files.

However, this process is difficult to follow in practice because it forces release authors to do work on behalf of other engineers. The people that know how to describe changes best are those that made the change, so they should be reponsible for updating changelogs.

With this commit, we now direct contributors to update changelog files directly as they make changes instead of placing them in the Changelog section of the PR description (which no longer exists). This commit adds documentation on how to do this effectively (much of which is now in the contributor-docs repo).

References

Progresses #4385.

Checklist

• I've updated the test suite for new or updated code as appropriate
• I've updated documentation (JSDoc, Markdown, etc.) for new or updated code as appropriate
• I've highlighted breaking changes using the "BREAKING" category above as appropriate
• I've prepared draft pull requests for clients and consumer packages to resolve any breaking changes