docs(sdk): add LLM Profiles guide synced with agent-sdk example

[enyst]

This PR adds a new SDK guide for LLM Profiles and syncs the example from agent-sdk.

Summary

• New page: sdk/guides/llm-profiles.mdx
• Explains profile storage, management with LLMRegistry (list/load/save), secret handling, and conversation persistence behavior with profile references via OPENHANDS_INLINE_CONVERSATIONS
• Adds an expandable example block that references the upstream script so docs remain in sync:
  • examples/01_standalone_sdk/31_llm_profiles.py
• Links the example profile JSON:
  • examples/llm-profiles/gpt-5-mini.json

Related PR(s)

• feat(llm): add LLM profiles software-agent-sdk#795 (feat(llm): add LLM profiles)

Co-authored-by: openhands openhands@all-hands.dev

@enyst can click here to continue refining the PR