Allow the Table of Contents to be expanded to show subsections and collapsed to hide subsections.

### Have you checked that your feature request isn't already filed?

- [X] I read through [FAQ](https://github.com/alshedivat/al-folio/blob/master/FAQ.md) and searched through the [past issues](https://github.com/alshedivat/al-folio/issues), none of which addressed my feature request.
- [X] Yes, I have checked that this feature request isn't already filed.

### Description & Motivation

Allow the Table of Contents to be expanded to show subsections and collapsed to hide subsections.
Also add support for sub-subsections

### Pitch

_No response_

### Alternatives

_No response_

### Additional context

_No response_