Make the outdated spec warning float at the top of the page

The outdated spec warning should scroll with the page so it is harder to miss if you just click a link deep in the spec and didn't start reading at the top.

> I googled for the specs, clicked the link and used Firefox's search function before even looking at the screen.

Start by locating the outdated spec warning in the matrix.org website and inspect how it is positioned when opening a deep link. Verify the warning remains visible while scrolling from a directly opened spec page, including in Firefox.

Written by the indexing model from the issue text.