docs: expand OpenAI-compatible endpoint recipes

[enyst]

• 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

• Stacked on docs: add OpenAI-compatible agent-server gateway guide #554 to expand the SDK Remote Agent Server OpenAI-compatible endpoint guide with copy/paste recipes for curl, the OpenAI Python SDK, the OpenAI JavaScript SDK, chat UI configuration, and voice/webhook-style conversation ID reuse.
• Documents the base URL, session API key, openhands_<profile_name> model naming, X-OpenHands-ServerConversation-ID state handling, and current limitations (non-streaming, final text only, no OpenAI tool-call surface).
• Adds visible links/notes from the SDK homepage, Remote Agent Server overview, and SDK architecture overview so users can find the endpoint.

Fixes OpenHands/software-agent-sdk#3540.

Validation:

• Ran a lightweight docs check that parses docs.json and verifies modified MDX code fences, Tabs components, and local links are balanced/resolvable.

This PR was created by an AI agent (OpenHands) on behalf of the user.

@enyst can click here to continue refining the PR