Search: Answer Questions Rather than show results v0

Many users ask questions of search boxes, and just returning what posts match is not a very effective method for having a question answering system. Example:

Auto-extracting this information is not feasible right now, but we can build a system which lets the site admin index frequently asked questions and have them point at posts or even create answers.

I think a way this could work is:
- User can attach questions to particular posts/pages which are then used for better search matching and inexactly answering questions
- For questions for which there are no posts to answer, we could define a custom post type that matches an answer to a question.

This is not a full system, but it could make it feel like it works well, and we would gather data which would let us eventually build a fully automated system.

For indexing, we will need to:
- define some post meta to store the questions
- index the questions appropriately

This is related to #8895

Thanks to @notnownikki for the discussions/explorations that led to this.

Start by reading this proposal alongside related issue #8895 and inspect the existing search and indexing entry points; the issue names no files or tests. Done would require an agreed design for question metadata, indexing, post or page links, and custom answers, followed by implementation and verification.

Written by the indexing model from the issue text.