Add "code quality" grouping to changelog docs

[tmlayton]

There were a few PRs (#524, #691, and #698) that touched a non-trivial amount of code, were done for code quality reasons, but did not have a changelog entry. This proved problematic when understanding what level of risk, thus manual testing needed to be done before releasing.

While we can go through the history of commits, it is more efficient to tell a clear story of impact in the changelog, even if not all changes are consumer facing.

[BPScott]

Nice

(Should all the groupings mentioned in the doc have a heading in unreleased.md?

[kaelig]

I like how it celebrates refactors and other unseen improvements.

I notice we don't have a category related to performance - and that's also something I'd love to see us celebrate / encourage / incentivize… would that fall under the same category?

[kvendrik]

🎉💯

[tmlayton]

Should all the groupings mentioned in the doc have a heading in unreleased.md?

Yes, I think they just got removed somewhere along the lines.

I notice we don't have a category related to performance - and that's also something I'd love to see us celebrate / encourage / incentivize… would that fall under the same category?

I’m good with adding a perf category, I think it makes sense to call out separately because it might not always make the source code "better" (easier to read, etc).

[tmlayton]

Will leave the suggestions for future PRs