Sticky support updates

This PR address a quick docs shortcoming and bulletproofs some CSS:

• Mentions the specific lack of support for position: sticky in Edge/IE11, including how those two browsers will render it as position: relative.
• Wraps the .sticky-top styles in a @supports (position: sticky) {} query so that only the browsers that properly support stickiness render it (thereby avoiding issues with relative styling in Edge/IE11).
• Wraps docs navbar styles in @supports query.

Follow up to #22751 (closed).