Update Moonshot provider docs for model picker, refresh, and .ai/.cn base URL

[edelauna]

Context

PR #857 on the main repo reimplements the Moonshot provider: live model picker fetched from the API, a "Refresh Models" button, support for both api.moonshot.ai and api.moonshot.cn base URLs, and new models (kimi-k2.6, kimi-k2.7-code, kimi-k2.7-code-highspeed).

docs/providers/moonshot.md is now out of date relative to these changes:

• Only mentions platform.moonshot.cn — no mention of the .ai endpoint / base URL dropdown now in settings.
• "Available Models" section just links out to Moonshot's own docs rather than describing the in-app model picker or the new Kimi K2.6/K2.7 models.
• No mention of the new "Refresh Models" button/flow for pulling the live model list from the API.
• Configuration steps don't mention selecting a base URL (api.moonshot.ai vs api.moonshot.cn) before entering the API key.

Ask

Update docs/providers/moonshot.md to cover:

1. Choosing between api.moonshot.ai and api.moonshot.cn base URLs.
2. The model picker now being populated dynamically via "Refresh Models," rather than a static list.
3. The current model lineup (kimi-k2.5, kimi-k2.6, kimi-k2.7-code, kimi-k2.7-code-highspeed) and their capabilities (context window, multimodal support).

Best done once #857 merges, so the doc matches shipped behavior.