doc: add guide for updating N-API API surface

This adds a new guide that outlines the principles and guidelines
for contributing a new API to the N-API surface. These guidelines
were formulated based on discussions in the API working group.

PR-URL: https://github.com/nodejs/node/pull/21877
Refs: https://github.com/nodejs/abi-stable-node/issues/301


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Vse Mozhet Byt <vsemozhetbyt@gmail.com>