- Fix typos in the README
- Don't echo all instructions in the linkcheck.sh shellscript; this is
noisy and unhelpful when developing locally. Verbose output is still
enabled in CI.
- Link to the index page for compiler docs, not `rustc_middle`. Note
that the compiler docs are not a guide - that's us!
- Remove outdated instructions about avoiding 429s. These should no
longer be occurring locally now that we're using caching.
- Consolidate info about `mdbook` and `mdbook-linkcheck` near each other