docs(llms): add OpenAI-compatible gateway section to Moonshot page

[Dawncoral]

• 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

Adds a short "Using an OpenAI-Compatible Gateway" section to the Moonshot provider page: the three Advanced settings (custom model with the openai/ prefix, base URL, API key) plus one verified gateway option for Kimi K3.

Why

The Moonshot page currently only covers the official platform, and its recommended model (kimi-k2-0711-preview) predates the current Kimi releases. Users who access Moonshot models through OpenAI-compatible gateways find no guidance on this page, while the same pattern is already documented elsewhere (for example, the LiteLLM proxy page).

Disclosure

I run Wallaby, the gateway named as the verified example. Happy to swap in a different example or leave the section generic if you'd prefer.

Testing

The openai/kimi-k3 + base URL configuration is what we run in Agent Canvas, verified end to end on 1.23.0 and 1.24.0 (chat, scheduled automations, webhook integrations).

[Dawncoral]

Happy to rebase or reshape the section if it doesn't fit the page's conventions — the gateway config in the diff was verified end-to-end against a live OpenHands setup, and I can attach a short screen recording of the flow if that helps review

[enyst]

I'm an AI agent based on Opus 5.5, helping Engel Nyst (@enyst) with project work.

Thanks for the disclosure. The generic part of this section is already on the OpenAI page (Using an OpenAI Proxy). It isn't Moonshot-specific, since any gateway works the same way. The remaining part names one gateway, which is the same question as #850, and maintainers have declined such vendor examples before (#504, #561, #619, and OpenHands/OpenHands#16857).

For the outdated kimi-k2-0711-preview recommendation, #634 already refreshes this page with Moonshot's own models. I'd suggest closing this in favor of that.