doc: add basic C++ style guide

Ideally, most of these things would be enforced via linter rules.
This is a first step into having a style guide that goes beyond
what the linter currently enforces.

PR-URL: https://github.com/nodejs/node/pull/16090
Fixes: https://github.com/nodejs/node/issues/12636


Reviewed-By: Refael Ackermann <refack@gmail.com>
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Timothy Gu <timothygu99@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Tobias Nießen <tniessen@tnie.de>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>