Add navigation loader and rename context provider

[conico974]

Introduce a navigation loader component to indicate loading state during navigation and rename the hash context provider to better reflect its purpose.

The loading state only appears after 400ms, pages that are fast enough or already loaded won't show the loading indicator.

[linear]

RND-7783 No loading spinner between route navigation

[changeset-bot]

⚠️ No Changeset found

Latest commit: 0573483

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[github-actions]

Summary of the deployments:

Version	URL	Status
Vercel	https://gitbook-v2-xg6mqawrt-gitbook.vercel.app	✅
Cloudflare	https://44ce2dbb-gitbook-open-v2-preview.gitbook.workers.dev	✅

Test content

Site	2v	2c
GitBook	https://gitbook-v2-xg6mqawrt-gitbook.vercel.app/url/gitbook.com/docs	https://44ce2dbb-gitbook-open-v2-preview.gitbook.workers.dev/url/gitbook.com/docs
E2E	https://gitbook-v2-xg6mqawrt-gitbook.vercel.app/url/gitbook.gitbook.io/test-gitbook-open	https://44ce2dbb-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	8 changed	Sep 23, 2025, 12:54 PM
v2-cloudflare (Inspect)	✅ No changes detected	-	Sep 23, 2025, 12:58 PM
v2-vercel (Inspect)	👍 Changes approved	5 changed	Sep 23, 2025, 12:56 PM