docs: adjust change log entries, and update HACKING.md to match

[tonyandrewmeyer]

Minor adjustments to the way that we record changes:

• Add more structure to CHANGES: group changes by the same types of groups as in our conventional commit messages (but with full words rather than abbreviations), and add the GitHub PR number that introduced the change
• Include all non-internal changes in CHANGES
• Remove "New in" notes from the API documentation (we want everyone to regularly update to the latest version, not pick a version that has whatever they need, and work hard to make this as easy as possible)
• Update HACKING to reflect these process changes

These changes are backported to ops 2.7, which is where the CHANGES file started.

[benhoyt]

Thanks for these updates. Just a couple of minor suggestions for CHANGES.md formatting.