Expand contributing "tip" in manual.adoc
Expand contributing "tip" in manual.adoc

- Add link to asciidoc website
- Explain how to create the generated adoc files from the source code
- How to run asciidoctor to generate the manual

rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer,rust-analyzer/rust-analyzer