docs(admins): add DaoXE OpenAI-compatible LLM gateway guide

[seven7763]

Description

Adds an admin guide for configuring DaoXE as an OpenAI-compatible multi-model gateway in Onyx.

DaoXE is not a first-party Onyx provider card. The guide walks through the existing OpenAI-Compatible path under Self-hosted & Custom:

• API base: https://daoxe.com/v1
• Bearer API key from the DaoXE account
• Fetch Available Models (account-scoped model IDs — no hardcoded public catalog)
• Multi-protocol note: Onyx uses Chat Completions here; DaoXE also exposes OpenAI Responses and Anthropic Messages for other clients
• Mainland China unavailability + use site-documented regional endpoint if needed

Also links DaoXE from the Language Models overview and the generic custom-provider page, and registers the page in docs.json.

Notes

• Docs-only; no product/code changes in onyx-dot-app/onyx
• I maintain DaoXE. OpenAI SDK examples: https://github.com/seven7763/DaoXE-AI
• Pattern matches existing gateway pages (OpenRouter / Bifrost / LiteLLM Proxy) while correctly using the OpenAI-Compatible UI card

Checklist

• Page under admins/ai_models/
• Nav entry in docs.json
• Overview card + cross-link from custom inference provider
• No secrets or hardcoded model ID / price tables