[docs] Table of contents scrolling behaviour is suboptimal

Problem: I've just come across https://pygments.org/docs/lexers/ which is a very long page. So I was happy to find a comprehensive table of contents in the sidebar – only to find that it does not scroll along when I scroll down, it only scrolls until the header is at the top of the page, and then stops. I have to scroll all the way to the end of the page (happy for the existence of the End button on my keyboard) when finally the ToC sidebar scrolls along to the end, and then I have to scroll up again to find the section I originally wanted.

Expectation: the ToC sidebar scrolls along as normal, fixed to the content, or is at least scrollable independently from the content.

This might be a bug in a sphinx theme used by the docs, but I didn't find out how that was called.

Start with the documentation page at https://pygments.org/docs/lexers/ and identify the Sphinx theme or documentation layout controlling the sidebar table of contents. Confirm the sidebar remains usable while scrolling a long page, either by following the content or by scrolling independently.