TOCs should be sticky

[fitojb]

Summary

Tables of contents disappear as you scroll the pages.

Process

1. Go to the Reference, Design, Examples, FAQ or Troubleshooting pages.
2. Click on an element from the table of contents or scroll down the page.

Current and expected result

The TOC disappears; it’s difficult to see where you are. The text on the left is still constrained to let a table of contents be on the right side.
The TOC is sticky in https://ubuntu.com/legal/data-privacy, for example.

[carkod]

I believe this should be supported in Vanilla, created an issue