Condense release notes

[rjyounes]

Agreed upon format:

One line for each item, with brief description and link to issue.
May include one-level nesting of bullets if an item has subitems significant enough to mention.
For major changes only: include an impact/migration statement.

Other decisions:

• Make sure every issue has a comment summarizing what we did and why, so readers don't have to read through a long comment thread to get the gist.

Implementation:

• remove release note template
• adjust any other documentation relating to release notes
• fix release notes for upcoming and previous releases