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

[Stephen Tozer via llvm-commits]

https://github.com/SLTozer commented:

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.

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