doc: Add initial documentation for N-API

Add the initial documentation for the N-API

This PR is a result of work in the abi-stable-node repo:
https://github.com/nodejs/abi-stable-node/tree/doc,
with this PR being the cumulative work on the documentation
sections in that repo with the following contributors
in alphabetical order:

Author: Arunesh Chandra <arunesh.chandra@microsoft.com>
Author: Gabriel Schulhof <gabriel.schulhof@intel.com>
Author: Hitesh Kanwathirtha <hiteshk@microsoft.com>
Author: Jason Ginchereau <jasongin@microsoft.com>
Author: Michael Dawson <michael_dawson@ca.ibm.com>
Author: Sampson Gao <sampsong@ca.ibm.com>
Author: Taylor Woll <taylor.woll@microsoft.com>

PR-URL: https://github.com/nodejs/node/pull/12549


Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Jeremiah Senkpiel <fishrock123@rocketmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>