Use a different CSS class to mark TOC levels as visible

[gabalafou]

Fixes #2156.

The underlying issue was that the CSS class visible is targeted by Bootstrap and results in the following rule:

visibility: visible !important;

which gets applied to the ul elements in the secondary sidebar. This overrides the visibility: hidden rule set on the sidebar container element, making the links focusable via the tab key.

How to test

1. On a desktop computer, go to a page in Read The Docs preview build. This page should have a right sidebar visible — i.e., the in-page table of contents.
2. Click near the bottom of the article and repeatedly press the tab key to observe how it moves through the links in the secondary sidebar.
3. Zoom the page, or narrow the width of your screen, until the secondary sidebar disappears.
4. Again, click near the bottom article before the article footer (which has two links to the previous and next articles)
5. Press the tab key and watch the focus move to those two previous/next footer links.
6. Keep pressing the tab key and observe that it does not go through any of the links in the sidebar. (Before this PR, the tab key would take you through the links in the sidebar, even though those links were not visible.)

[gabalafou]

For reference, the PR that added the visible class to the ul elements in the page TOC:

• ✨ Using sphinx toc functions #219