[stable8] chore(docs): add missing backports around supported versions and links

[ShGKme]

☑️ Resolves

• Partial manual backport of: chore: default branch is main and Vue 2 branch is stable8 #6697
• Partial manual backport of: docs: v9 will only provide visual compatibility with Nextcloud 32+ #7287
• Partial manual backport of (the last commit): chore: revert breaking changes and keep Nextcloud 31 support #7353
• Note, some changes were backported in commits:
  • 7ba00f8
  • 5a2c048
  • 014c93d

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 44.88%. Comparing base (87096aa) to head (52b79cb).
⚠️ Report is 4 commits behind head on stable8.

Additional details and impacted files

@@           Coverage Diff            @@
##           stable8    #7773   +/-   ##
========================================
  Coverage    44.88%   44.88%           
========================================
  Files          180      180           
  Lines         4465     4465           
  Branches      1115     1115           
========================================
  Hits          2004     2004           
  Misses        2368     2368           
  Partials        93       93           

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.