rust.css: Make it more clear when code spans are links
Add some notes about macro scoping
Use relative doc URLs in top-level guides
Update some old references to rust.md
Add a red-box warning to the macros guide
Add a guide to compiler plugins

Fixes #16983.
Link plugins guide from elsewhere
Disable the Roman numerals test on Android due to cross-compile issues
auto merge of #16995 : kmcallister/rust/plugin-tutorial, r=alexcrichton

@steveklabnik, are you interested in looking this over?