LCORE-1037: update BYOK and RAG guides to use lightspeed-stack config

Replace all run.yaml references with lightspeed-stack.yaml byok_rag
configuration. Users should no longer edit run.yaml directly — the
Lightspeed Stack service auto-generates Llama Stack config at startup
from the byok_rag and rag sections in lightspeed-stack.yaml.

- Rewrite BYOK guide Step 4 to use byok_rag as primary config path
- Add field reference table for all byok_rag options
- Rewrite RAG guide vector store sections with byok_rag examples
- Replace full run.yaml config examples with lightspeed-stack.yaml format
- Add embedding_model field to BYOK example config
- Update Step 3 to reference byok_rag embedding_model field

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: are-ces