Switch main DocumentView to flex layout

[zenoachtig]

Making the main view a flex layout instead of a single-column grid will make content easier to truncate content horizontally. Blocks that need it (like columns) can still use their own grid.

[changeset-bot]

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Latest commit: 96079ff

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-5e510r8xy-gitbook.vercel.app	✅
Cloudflare	https://cdc8898e-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-5e510r8xy-gitbook.vercel.app/url/gitbook.com/docs	https://cdc8898e-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-5e510r8xy-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://cdc8898e-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.gitbook.io/test-gitbook-open

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Build	Status	Details	Updated (UTC)
customers-v2 (Inspect)	👍 Changes approved	1 changed	Dec 16, 2025, 10:35 PM
v2-cloudflare (Inspect)	👍 Changes approved	13 changed	Dec 16, 2025, 10:39 PM
v2-vercel (Inspect)	👍 Changes approved	20 changed	Dec 16, 2025, 10:39 PM