Incorrect syntax / use for heading in sidebar

[ChrisXKeroack]

Describe the bug
Sidebar uses a heading without declaring a heading level

To Reproduce
Steps to reproduce the behavior:

1. https://openfl.readthedocs.io/en/latest/index.html
2. Examine the code for the 'CONTENTS:' text

notice: Sidebar declares a heading for ‘contents’ via role=heading but doesn’t set a heading level

Expected behavior
add aria-heading="2"

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).

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.

Note: the ARIA 1.1 specification allowed the use of role=heading without an accompanying aria-level attribute, and would default to a heading level of 2 if aria-level was missing. The ARIA 1.2 specification forbids that and requires an aria-level attribute.