Display updates blocks headings in page outline

[clairechabas]

Add support for headings within update blocks to be displayed in the page outline.

[linear]

RND-10337 Include headers from Updates block in page outline

[changeset-bot]

⚠️ No Changeset found

Latest commit: 160241b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel		❌
Cloudflare		❌

Test content

Site	2v	2c
GitBook	/url/gitbook.com/docs	/url/gitbook.com/docs
E2E	/url/gitbook.gitbook.io/test-gitbook-open	/url/gitbook.gitbook.io/test-gitbook-open