doc: linkable commit message guidelines

Put the commit guidelines themselves under a heading to be more
prominent, and to allow linking directly to them (instead of the section
on how to use git).

Link the pull request template to the guidelines, so contributors can
find them.

PR-URL: https://github.com/nodejs/node/pull/11792


Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Luigi Pinca <luigipinca@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: James M Snell <jasnell@gmail.com>