Feature: ARIA-compatible collapsible navigation for Sphinx theme #1785

Describe the enhancement or feature you would like

The discussion about keyboard shortcuts #1784 (comment) revealed that there are might expectations about arrow keys to the collapsible tree items.

From ARIA Guide.

Just as familiar visual styling helps users discover how to expand a tree branch with a mouse, ARIA attributes give the tree the sound and feel of a tree in a desktop application. So, screen reader users will commonly expect that pressing the right arrow key will expand a collapsed node.

The implementation should add a local keyboard shortcut to left/right keys similar to global one defined for page navigation here https://github.com/sphinx-doc/sphinx/blob/cc7c6f435ad37bb12264f8118c8461b230e6830c/sphinx/themes/basic/static/doctools.js#L106 that should do the ARIA thing.

I suggest we find real ARIA users who can define what the right ARIA thing should do and help test it.

Describe alternatives you have considered

No response

Additional context

• https://webaim.org/techniques/keyboard/