Pin the top level menu

[razvan]

The top level menu should not be hidden when scrolling down. This forces the reader to scroll up when navigating between concepts and operator docs which becomes tedious quickly.

[fhennig]

@lfrancke and @soenkeliebau also think it should be pinned, so we'll do it.

[Techassi]

Yes, I also like a pinned nav bar!