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 nodejs#26929.

Refs: nodejs#26929

Author: addaleax

CPP_STYLE_GUIDE.md

@@ -1,5 +1,8 @@
 # C++ Style Guide
 
+See also the [C++ codebase README](src/README.md) for C++ idioms in the Node.js
+codebase not related to stylistic issues.
+
 ## Table of Contents
 
 * [Guides and References](#guides-and-references)