improve docs responsiveness

 - Insert docs CSS into origin HTML rather than being loaded by SJS,
   to have them fetched faster (and reduce FOUC)

 - Load search indexes in the background, so they don't
   delay display of the search widget.

 - Simplify search index update logic to prevent overlapping updates

 - Discard non-latest search results (if a later query finishes quicker
   than an earlier one, the earlier results will never be shown)