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: #26929

PR-URL: #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>

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)

doc/guides/contributing/pull-requests.md

@@ -121,7 +121,9 @@ in the API docs will also be checked when running `make lint` (or
 use `REPLACEME` for the version number in the documentation YAML.
 
 For contributing C++ code, you may want to look at the
-[C++ Style Guide](../../../CPP_STYLE_GUIDE.md).
+[C++ Style Guide](../../../CPP_STYLE_GUIDE.md), as well as the
+[README of `src/`](../../../src/README.md) for an overview over Node.js
+C++ internals.
 
 ### Step 4: Commit