docs: add anomaly detection window functions reference (EN + ZH)

[Copilot]

Documents the three statistical anomaly-scoring window functions added in GreptimeTeam/greptimedb#7681.

What's Changed in this PR

New docs/reference/sql/functions/anomaly.md (EN + ZH) covering:

• anomaly_score_zscore(val) — Z-Score: |x − mean| / stddev, min 2 samples
• anomaly_score_mad(val) — MAD: |x − median| / (MAD × 1.4826), min 3 samples
• anomaly_score_iqr(val, k) — IQR/Tukey Fences: distance beyond Q1−k×IQR / Q3+k×IQR, min 3 samples

Each function documents signature, formula, minimum sample requirements, and degenerate-case behavior (NULL / 0.0 / +inf). Page includes a full end-to-end example and subquery pattern for filtering anomalous rows:

SELECT * FROM (
    SELECT host, ts, val,
        ROUND(anomaly_score_mad(val) OVER (
            PARTITION BY host ORDER BY ts
            ROWS BETWEEN UNBOUNDED PRECEDING AND CURRENT ROW
        ), 2) AS mad
    FROM sensor_data
) WHERE mad > 3.0;

• Linked from functions/overview.md (EN + ZH)
• Added reference/sql/functions/anomaly to sidebars.ts

Checklist

• Please confirm that all corresponding versions of the documents have been revised.
• Please ensure that the content in sidebars.ts matches the current document structure when you changed the document structure.
• This change requires follow-up update in localized docs.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>Update docs for feat: implements anomaly_score_iqr, anomaly_score_mad etc.</issue_title>
<issue_description>A document change request is generated from GreptimeTeam/greptimedb#7681</issue_description>

<agent_instructions>Write the documents for this PR GreptimeTeam/greptimedb#7681, including chinese and en version in nightly.</agent_instructions>

Comments on the Issue (you are @copilot in this section)

• Fixes Update docs for feat: implements anomaly_score_iqr, anomaly_score_mad etc. #2330

---

🔒 GitHub Advanced Security automatically protects Copilot coding agent pull requests. You can protect all pull requests by enabling Advanced Security for your repositories. Learn more about Advanced Security.

[cloudflare-workers-and-pages]

Deploying greptime-docs with    Cloudflare Pages

Latest commit:	7844158
Status:	✅  Deploy successful!
Preview URL:	https://439472ff.greptime-docs.pages.dev
Branch Preview URL:	https://copilot-update-docs-anomaly.greptime-docs.pages.dev

View logs

[killme2008]

LGTM

[fengjiachun]

LGTM