Lead each release with highlights in What's new, the website changelog and release notes (#9281)

Every release entry in CHANGELOG.md now leads with highlights: an unheaded
list above the first `###`. The app's What's new sheet, the website
changelog and the GitHub release body show the highlights and fold the
sections, the full record, behind one control. An entry without highlights
keeps its sections in view. docs/release.md carries the writing rules.

Installed apps fetch CHANGELOG.md from main and parse it with the parser
they shipped with, so the two heading rules are a compatibility surface:
`##` starts a release, `###` starts a section, and headings inside fenced
code are text. The app, website and release-script parsers now split an
entry the same way and each runs the shared cases in
scripts/test-support/changelog-structure.json. The release-script tests,
which no CI job ran before, are part of "Validate CI contracts".

Adds the 0.16.0-beta.1 entry (en and zh-CN) ahead of the release.

.github/workflows/ci.yml

@@ -56,7 +56,7 @@ jobs:
           filters: .github/ci-paths.yml
 
       - name: Validate CI contracts
-        run: node --test scripts/ci-workflow.test.mjs scripts/daemon-launch-contract.test.mjs scripts/sync-fdroid-changelogs.test.mjs scripts/upload-release-assets.test.mjs
+        run: node --test scripts/ci-workflow.test.mjs scripts/daemon-launch-contract.test.mjs scripts/changelog-utils.test.mjs scripts/sync-fdroid-changelogs.test.mjs scripts/sync-release-notes-from-changelog.test.mjs scripts/upload-release-assets.test.mjs
 
   quality:
     name: quality