[Precursor] Precursor Changelog Entry for Launch 13 July

[marinaelmore]

Add the Cloudflare Challenges changelog post for Precursor session-based detection, including links to the launch blog and Precursor docs. Include an animated dashboard walkthrough asset for enablement context.

Summary

Changelog for Precursor

Screenshots (optional)

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.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

[cloudflare-docs-bot]

Review

⚠️ Part of the review could not complete in commit b3113f6 — it will retry on the next push.

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Style Guide Review

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