Collapsable table of contents side bar in documentation

[chris-ashe]

In its current version the documentation has a side bar table of contents for the current page with each clickable header name and indentation depending on their level.
With lots of the pages now being populated it would be good to see if these section can be collapsable based on their level to prevent cluttering and ease navigation through the page

[chris-ashe]

squidfunk/mkdocs-material#695