Add initial architecture blueprint for Zoekt

What does this MR do and why?

This is an architecture blueprint to describe the Zoekt integration we are working on in !105049 (merged) . The MR is still a WIP and there are important architectural details we'd like feedback on early.

Rendered markdown can be viewed at https://gitlab.com/gitlab-org/gitlab/-/blob/zoekt-blueprint/doc/architecture/blueprints/search/code_search_with_zoekt.md

How to set up and validate locally