Better docs search

With #123 merged, we should consider using [RtD's server-side search](https://docs.readthedocs.io/en/stable/server-side-search.html) powered by Elasticsearch, as an alternative to the sphinx-powered on-page search (see the [search](https://docs.readthedocs.io/en/stable/config-file/v2.html#search) option).

Read the linked Read the Docs server-side search documentation and the linked configuration search option, then review the context from merged issue #123. The issue does not name repository files, tests, or acceptance criteria; completion would require deciding whether and how to replace the Sphinx-powered on-page search with Read the Docs server-side search.

Written by the indexing model from the issue text.