Larger h1 to enable three levels of headings.

<img width="508" alt="Screenshot 2024-05-24 at 07 38 31" src="https://github.com/enso-org/enso/assets/1047859/53e77040-30c2-4ed0-bfb5-81d4a703a565">

Fixes #10051.

# Important Notes
Refactored Lexical styling.