doc: add Table of Contents to Cpp style guide

The Cpp style guide is growing. IMHO, a Table of Contents makes
it easier to navigate.

PR-URL: https://github.com/nodejs/node/pull/17052


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>