Add fixed positioning to top level navbar

Annabel Dunstone Grayrequested to merge
30975-fixed-top-nav-POC into master

Adds fixed positioning on top level nav

Need to double-check:

  • All fixed sidebars become fixed at the right position
  • Any other fixed elements (MR tabs) become fixed at the right position
  • Enough top padding for the content area now that the navbar has been taken out of the flow of elements

Closes #30975 (closed)

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