doc/dev: Add code intelligence development documentation
Created by: efritz
This PR creates a new section for code intelligence developers:
- General architecture
- How to deploy (this will become more relevant once we have the indexer service outside of k8s)
- Sequence diagrams/discussion for code intel queries in the backend
- Sequence diagrams/discussion for index processing
- Sequence diagrams/discussion for how code intel extensions resolve data
Each of these contain references to relevant code to help onboarding. The last point also contains a search query appendix, which has been a point of repeated questions in the past. Hopefully that will be helpful outside of the code intel team as well!