docs(rum): document AI natural-language query in the Explorer

[Fiona2016]

What

Documents the new AI natural-language query feature in the RUM Explorer.

Adds an "AI 自然语言查询 / AI natural-language query" section to rum/explorer/data-query.mdx (zh + en), covering:

• Magic-wand entry into AI query mode (highlighted input border).
• Describe-and-Enter conversion: plain-language description (zh/en) → standard DQL, with a worked example.
• Preview / Apply / Undo flow, including regenerating by refining the description.
• Append-not-replace behavior: AI conditions are appended to an existing query; event-type switch prompt; server-side field allowlist (no need to know field names).
• A tip to describe intent in natural language rather than hand-writing DQL in the AI input.

Why

The feature (POST /rum/query/translate) is being released to production — backend fc-rum#117, gateway fc-pgy#626, frontend fc-saas-rum#241 — and had no user-facing documentation.

Notes

• Added as a section within the existing query-syntax guide (no docs.json navigation change).
• mint broken-links passes.
• Example query verified against the feature's own end-to-end test cases.