Add ARIA labels to improve documentation accessibility

[Privat33r-dev]

Documentation

According to the Lighthouse report, select elements do not have labels.

These select elements are rendered in the JS in Doc/tools/static/rtd_switcher.js.

The issue was originally outlined on the Discourse (6th point).

Linked PRs

• gh-125933: Add ARIA labels to select elements in the version switcher #125934
• [3.13] gh-125933: Add ARIA labels to select elements in the version switcher (GH-125934) #125938
• [3.12] gh-125933: Add ARIA labels to select elements in the version switcher (GH-125934) #125939