Add ReST/Sphinx usage guide to our contributing documentation

[dbkinder]

New contributors to the project (and its documentation) could use practical guidelines on ReSTructuredText and Sphinx directives, and our project conventions. Early versions of our technical docs included some of this information but was removed. Bring that back and update with current conventions and practical issues we see in documentation contributions (e.g., conventions on indenting and heading underlines, use of zephyr-specific directives, and known issues and workarounds).

[dbkinder]

• Document use of .. zephyr-app-commands directive
• Document code-block directive language options (including dts)