Spec Redesign: Support filtering the sidebar

It would be very nice, if the sidebar was searchable, so that I can just search for "message" in the sidebar and click through all sections with that in its title. (Similar to how you can filter rooms in Element). That makes it much easier to jump between sections and I wanted to have that for a long time and the new spec design makes that something finally in reach!

The issue names no files or tests. Start by locating the specification site's sidebar implementation and reviewing how its sections are generated; compare the requested behavior with Element's room filtering. Done means users can filter sidebar entries by title and navigate through matching sections.

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