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

PR-URL: https://github.com/nodejs/node/pull/31149


Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anto Aravinth <anto.aravinth.cse@gmail.com>
Reviewed-By: Beth Griggs <Bethany.Griggs@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>