Support new sidebar title page option

[clairechabas]

Support page's sidebar custom title

⚠️ Pending API update: this PR is awaiting for the API update PR to be merged and for the API to be bumped. That's it's not building correctly for now. It'll be merged after all this is done.

[linear]

RND-8419 Add option to set sidebar title that's different than page title

[changeset-bot]

⚠️ No Changeset found

Latest commit: 69732c6

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