[DLP] Add Test scan documentation and changelog

[Jose-Maria-Martins]

Summary

Adds documentation for DLP Test scan, a new dashboard tool that lets administrators test text, files, and HAR data against DLP profiles without sending traffic through Gateway.

New page:

• src/content/docs/cloudflare-one/data-loss-prevention/test-scan/index.mdx — how-to page covering procedure, results reference, worked example, troubleshooting, and limitations.

Cross-links added to existing pages:

• data-loss-prevention/index.mdx — pointer under Troubleshooting section
• dlp-profiles/index.mdx — recommendation after custom profile walkthrough
• dlp-policies/index.mdx — diagnostic tip inside "Test DLP policy" section
• detection-entries/configure-detection-entries.mdx — new "Test a detection entry" section
• troubleshoot-dlp.mdx — pointer before existing symptom sections

Changelog:

• src/content/changelog/dlp/2026-08-21-dlp-test-scan.mdx

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.

[cloudflare-docs-bot]

Review

✅ No issues found in commit 71c4023.

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