docs(changelog): 5.10.0 — tray zones, @ext.menu_item, and the BackButton that never shipped

The release workflow builds its GitHub Release notes by pulling the
CHANGELOG section whose heading matches the tag, so a missing section
means a release page reading "Release v5.10.0" and nothing else.

Two things worth noting in this entry beyond the new surfaces:

* v5.9.23 (ui.BackButton) was committed but never tagged, so it never
  reached PyPI — PyPI is still serving 5.9.22. It ships inside 5.10.0
  rather than disappearing from the package's history.
* the tray/menu sections document WHY account/footer are refused for
  contributions, because that is the part a future contributor will
  otherwise read as an arbitrary restriction and try to remove.

Verified the workflow's awk extractor against this file: 68 lines, stops
cleanly before the 5.9.19 heading.
