[llvm] [Docs] Fix large vertical space at top of pages (PR #81599)

[J. Ryan Stinnett via llvm-commits]

jryans wrote:

> There are already styles for `body` and `div.sphinxsidebar` - is there a reason for adding these changes under new selectors? There is the comment, but it doesn't look like we actually use these kind of explanatory comments in css regardless.

Purely for grouping together with the comment, which I thought might be clearer for the next CSS archaeologist that passes through. You're right that's not the style used here though. I think the bulk of this file is providing an appearance and not "fixing quirks" like this, so that's perhaps on point in favour of what I've done here.

Anyway, I'm happy to remove the comment and fold into exist rule sets if that's the preferred way to go! 🙂 

https://github.com/llvm/llvm-project/pull/81599