No programmatic description of expandable elements in sidebar as being expandable, until/unless they are expanded

[ChrisXKeroack]

Describe the bug
Screenreader user won't hear expandable elements in sidebar are expandable

To Reproduce
Steps to reproduce the behavior:

1. start a screenreader
2. https://openfl.readthedocs.io/en/latest/index.html
3. TAB to 'Manual'

Notice: Element is not described as expandable

Expected behavior
Screenreader to be able to tell me that this is expandable and currently collapsed, e.g. via support for aria-expanded

Screenshots

Desktop:
This issue is not OS or browser specific, but for tracking purposes, testing was done on Win11, in Edge Version 114.0.1823.58 (Official build) (64-bit), and the screenreader in use was the common 3rd party screenreader NVDA (http://www.nvaccess.org).

Smartphone:
This issue is not device, OS, or browser specific.

Additional context
This is an accessibility issue, please annotate this bug with the 'accessibility' label.

[fstrr]

This is an issue with the Read The Docs platform.