- Use a Sphinx/Myst {include} directive to pull the text from all previous
RELEASE files into the output, which makes the website sidebar index work
properly.
- Exclude the RELEASE files from the files given to RTD to publish, they
aren't needed now and generate warnings if included.
- Updated the intro paragraph in all RELEASE files.
Adds new-notes files for the PRs already merged.
Future conflicts in the now-deleted RELEASE_NOTES.md file must be
turned into new-notes/PR-nnn.md files when resolving them.
