Improve changelogs

[kaylareopelle]

It's difficult for me to trace the exact change related to a given release. I think we can make some updates to the structure of the changelog to make things more user-friendly.

Some improvements ideas:

• Add a link to the PR(s) that caused the change
• Use the PR title as the changelog title
• Include a description for the change (perhaps we can have PR descriptions or content beneath a specific heading determine the description)
• Have an overall release changelog for contrib as a whole (which PRs have been released? what gems did they impact?)
  • Maybe this looks like a tag that's easy to filter on PRs? Maybe something like this exists and I'm now aware?

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