Split manual.adoc into markdown files, one for each chapter.
For the parts of the manual that are generated from source code doc
comments, update the comments to use markdown syntax and update the
code generators to write to `generated.md` files.
For the weekly release, stop copying the .adoc files to the
`rust-analyzer/rust-analyzer.github.io` at release time. Instead,
we'll sync the manual hourly from this repository.
See https://github.com/rust-analyzer/rust-analyzer.github.io/pull/226
for the sync. This PR should be merged first, and that PR needs to be
merged before the next weekly release.
This change is based on #15795, but rebased and updated. I've also
manually checked each page for markdown syntax issues and fixed any I
encountered.
Co-authored-by: Lukas Wirth <lukastw97@gmail.com>
Co-authored-by: Josh Rotenberg <joshrotenberg@gmail.com>
