## Problem
This page had many dead links, and was confusing for folks looking for
documentation about our product.
Closes: https://github.com/neondatabase/neon/issues/8535
## Summary of changes
- Add a link to the product docs up top
- Remove dead/placeholder links
Reorganize existing READMEs and other documentation files into mdbook
format. The resulting Table of Contents is a mix of placeholders for
docs that we should write, and documentation files that we already had,
dropped into the most appropriate place.
Update the Pageserver overview diagram. Add sections on thread
management and WAL redo processes.
Add all the RFCs to the mdbook Table of Content too.
Per github issue #1979
