doc: use code markup/markdown in headers

This also allows us to remove backslash escaping for [ and ] inside of header code, which makes the bare markdown more readable.

Refs: https://github.com/nodejs/node/pull/31086

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines