Allow users to filter search results by content types

As we've moved our audits to HTML some users have said they have a difficult time finding the binder page for an audit in search results, because when they search for an agency name they will also get affiliated info details pages, press releases, and other information, even if they filter for content by our agency.

I'd like to see users be able to filter by specific content types in their search results. For example, if someone selected to filter by "Audit" they would only get results that included binders that were marked as being audits.

The issue names no files, tests, or entry points. Start by locating the search-results filtering flow and how audit binders are marked as content types, then confirm the supported types and expected filter behavior with maintainers. Done means users can select a type such as Audit and see only matching results.

Written by the indexing model from the issue text.