docs(llm): document OCI Generative AI setup

[Xiaowen-Yang]

• I have read and reviewed the documentation changes to the best of my ability.
• If the change is significant, I have run the documentation site locally and confirmed it renders as expected.

Summary of changes

• Added setup instructions for using OCI Generative AI with OpenHands.
• Documented the required model ID, OCI region, Generative AI project OCID, and API key.
• Added links to the relevant OCI documentation for finding supported models and configuring project access.
• Clarified how these values map to the OCI Generative AI fields in the OpenHands LLM settings UI.

Related to OpenHands/OpenHands#17512.

Related implementation:

• feat(llm): add OCI Generative AI provider software-agent-sdk#5186
• feat(llm): add OCI Generative AI setup OpenHands#17543