docs: clarify Higress AI route matching

[RerankerGuo]

Summary

• add English and Chinese FAQ entries for custom Higress AI routes that never match
• explain that an unconstrained default-ai-route can match all model requests
• document that multiple AI routes should each use explicit modelPredicates, including the default route

Tests

• git diff --check

Addresses #867.

[RerankerGuo]

@shiyiyue1102 Hi! This PR documents the fix for #867 (custom AI routes never matching). It explains that an unconstrained default-ai-route can match every model request, and recommends setting explicit modelPredicates on every route (including the default) when running multi-route setups. English + Chinese FAQ entries.

translate is green and the branch is MERGEABLE. Docs-only. Could you take a look?