WIP: enable section indexes by default, allow deeper nesting in sidenav

[ntno]

• refactor sidenav to use recursion
• add option to allow for deeper sidenav nesting
• doc reorganization
• breaking change version bump

TODO:

• consider: add back in feature where an active page does not display as a clickable link in the sidenav
• review documentation that refers to ^
• unit tests
• fix or write up TODO issue for the bug where there is an empty ul element
• consider: prefix config.extra variables with mkdocs-terminal
  • consider: do they need to be caps?
• update docs so that "navigation.side.indexes" is standard
• remove references to "navigation.side.indexes" in code
• check what happens if you give a regular page the title "Index" -> does it get skipped? should only get skipped if it is inside a section(?)

Description

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Testing

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Checklist

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[ntno]

relates to #137