Clean up introduction

Was: "Clean-up based on what is (not) supported and framed well enough yet"

• Stop rendering RA stuff, keep in git repo. Until we're ready to support RA.
• Clean-up introduction, so that it's just a single page framing docs.system-transparency.org. More extensive introduction is TODO / later.
• Flatten the subcategories -- and no empty things.
• Frame "Learn more" more appropriately, keep the content rendered. Useful related reading.
• Contribute tab -- seems more like git README.

Updating the issue name to be more concrete (2024-05-05).