Add semantic documentation search for repos/wikis

Problem to solve

This is inspired by #477708

Currently, it's quite hard to find documentation from different repositories. You can use Advanced Search and/or Exact code search, but it's still not ideal for documentation.

Proposal

I believe that we should add semantic (hybrid) search to documentation hosted in repositories (for example, markdown files in /doc directory) as well as to wikis, which is essentially markdown files.

We could allow configuring the folders we need to index on namespace and project levels. We should also allow the instance/namespace admins to control this functionality since not all customers are comfortable sending their data to the LLM.

I suggest we use Advanced Search to implement this. Right now, we already have hybrid search enabled for issues, so it should be pretty similar. We can call this new index gitlab-production-customer-documentation.

This work can also be used for generating code embeddings later (if needed) since we'll need to update https://gitlab.com/gitlab-org/gitlab-elasticsearch-indexer