Docs: left-side component does not scroll

[ipopescu]

The docs left-hand component, which shows various subcategories of articles, does not scroll independently of the rest of the page most of the time. Most of the time, I have to scroll to the bottom of the whole page to see the bottom of that container. The scroll appears sometimes if we have sub-headings 3 levels down, but even then it is not consistent.

Screen capture: https://github.com/casper-network/docs/assets/4185994/2c558d90-2ff9-45ab-a558-4d34cca1da51

@Adrian-Wrona FYI.