doc: enable visual code indication in headers

This enables the grey background for inline code in headers.

Refs: https://github.com/nodejs/node/pull/31460#issuecomment-577154740

PR-URL: https://github.com/nodejs/node/pull/31493


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Geoffrey Booth <webmaster@geoffreybooth.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>