Reduce header depth and remove colon for release notes

[simonjjones]

Summary

This change reduces the header depth of release notes headers in the markdown format, removes the colon from the end of these headers and adds a new line between these headers and the corresponding content. All of these changes have been made at the request of documentation writers.

Use Cases

This will slightly change the appearance of release notes which are published alongside each of our releases

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.
• I'm happy with the commit history on this PR (I have rebased/squashed as needed).

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: simonjjones / name: Simon Jones (318b1aa)

[robdimsdale]

Closing and re-opening to re-trigger CLA bot