[Code Embeddings] Add developer documentation for Indexing Pipeline

Context

ActiveContext has some generic usage docs, but this does not cover everything for the Code Embeddings Indexing and its use in Duo Chat

We need to add developer docs specific to:

• Code Embeddings Indexing Pipeline - this issue
• Duo Chat Integration - addressed in #571912

References

Code Embeddings Indexing Pipeline setup links

• Setup for gitlab-elasticsearch-indexer
• Setup for Rails / Index state management

Other relevant links

• Code Embeddings Blueprint