Single search interface across all docs sites

It would be a huge improvement to have a single search interface across:
- developer.holochain.org
- rust docs
- js client docs

Where search results link to their original site.

It may be possible with agolia, if its not possible with our current search library.

Begin by reviewing the current search setup for developer.holochain.org, the Rust docs, and the JS client docs; the issue does not name files, tests, or the current search library. Define the cross-site result and original-site linking behavior, then verify the selected approach across all three sites.

Written by the indexing model from the issue text.