docs: rewrite OCI page with native Cohere Python SDK documentation

Complete rewrite of the OCI documentation page to cover the new
OciClient and OciClientV2 classes:

- Installation (pip install cohere[oci])
- Quick start: V2 chat, V1 chat, embeddings, V2/V1 streaming
- All authentication methods (config, profile, session, direct,
  instance principal, resource principal)
- V1 vs V2 API comparison table
- Tool use, vision, and embed v4 snippets
- Supported features table
- End-to-end example: embed → search → grounded chat → tool calling
  (multi-turn) → vision → streaming

All code snippets verified against live OCI Generative AI.

Author: fede-kamel