Styling: Markdown header levels

[nomeata]

The HTML output for release notes (rendered from Markdown) don’t use h2, h3 etc, making it hard to understand the document structure (e.g. when the highlights end and the first section starts.)

https://lean-lang.org/doc/reference/latest/releases/v4.21.0/

[david-christiansen]

Luckily for us, @jrr6 improved the Markdown mode so that it can handle headers without faking them, so this should be relatively simple to fix.