docs: add mpv.launchMode to config docs, add changelog:docs generator, format

- Document the new mpv.launchMode option in the configuration docs page
- Add changelog:docs command to auto-generate docs-site/changelog.md from root CHANGELOG.md
- Add breaking changes support to the changelog fragment generator
- Fix docs-sync test to only compare current minor release headings
- Apply prettier formatting to source files

changes/README.md

@@ -12,10 +12,21 @@ area: overlay
 - Added auto-pause toggle when opening the popup.
 ```
 
+For breaking changes, add `breaking: true`:
+
+```md
+type: changed
+area: config
+breaking: true
+
+- Renamed `foo.bar` to `foo.baz`.
+```
+
 Rules:
 
 - `type` required: `added`, `changed`, `fixed`, `docs`, or `internal`
 - `area` required: short product area like `overlay`, `launcher`, `release`
+- `breaking` optional: set to `true` to flag as a breaking change
 - each non-empty body line becomes a bullet
 - `README.md` is ignored by the generator
 - if a PR should not produce release notes, apply the `skip-changelog` label instead of adding a fragment