Search: add 404 search recommendation block

Add a recommendation block that is displayed on 404 pages and can function as recommendations.

This could be a fork of the existing Related Posts block to start and we just need to add an option to the search API to treat the query string more as context rather than a requirement when matching.

Start by locating the existing Related Posts block and the search API entry point, then trace how 404 pages render blocks. Define the recommendation block’s expected display and verify that the API can use the query string as context rather than a strict matching requirement; done means recommendations appear on 404 pages without requiring an exact query match.

Written by the indexing model from the issue text.