src,doc: add C++ internals documentation

This aims to help explain some of the internal patterns and utilities
that we use. It is by no means exhaustive, and suggestions for
additions are welcome.

Some of this is based on the existing work from #26929.

Refs: https://github.com/nodejs/node/pull/26929

PR-URL: https://github.com/nodejs/node/pull/30552


Reviewed-By: Sam Roberts <vieuxtech@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Jiawen Geng <technicalcute@gmail.com>
Reviewed-By: David Carlier <devnexen@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Gireesh Punathil <gpunathi@in.ibm.com>
Reviewed-By: Denys Otrishko <shishugi@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>