docs: release notes for Renovate v39

[HonkingGoose]

Changes

• Write release notes for Renovate v39, based on the 39.0.0 release notes on GitHub
  • Group similar parts
  • Add headings
  • Use lists
  • Explain why we made certain changes
• Enable announcement bar feature in the MkDocs config file
• Update the announcement bar text, so it mentions v39
• Explain how a user can allow squash merges on their GitHub repo, and link to the GitHub Docs

Context

The hosted app now uses Renovate v39. So we should put nice release notes in the docs. 😄

Documentation (please check one with an [x])

• I have updated the documentation, or
• No documentation update is required

How I've tested my work (please select one)

I have verified these changes via:

• Code inspection only, or
• Newly added/modified unit tests, or
• No unit tests but ran on a real repository, or
• Both unit tests + ran on a real repository

[HonkingGoose]

Thank you for applying the review suggestions for me! 😄 I think we're good to go. I'm marking this PR as ready for review, and will let you review and then merge.

[renovate-release]

🎉 This PR is included in version 39.15.3 🎉

The release is available on:

• GitHub release
• npm package (@latest dist-tag)
• 39.15.3

Your semantic-release bot 📦🚀