[BIZ-1892] Attempt Subnavigation Feature

This pull request adds subnavigation in a sustainable way and switches over the theme to be consistent with our private docs. The standard 'ReadTheDocs' theme for mkdocs does not handle subnavigation effectively-- autoexpanding all entries, so we have switched over to the Material theme used by the private docs.

Jira Tickets: https://tasks.opencraft.com/browse/BIZ-1892

Sandbox: https://handbook.opencraft.com/en/fox-biz-1892-subnavigation/

Reviewers

• @Agrendalath