feat(view-transitions): implement View Transition API for smoother pa…

[soheilnikroo]

…ge navigation

• Added support for the View Transition API to enable animated transitions between routes during client-side navigation.
• Updated PendingNavigation to utilize the new API, allowing for smoother visual transitions.
• Introduced viewTransitionUtils for managing view transitions and added corresponding CSS for animations.
• Added documentation and a guide on how to enable and customize view transitions in the Docusaurus configuration.
• Updated tests to cover new functionality and ensure proper behavior of view transitions.

This feature is experimental and can be enabled via the experimental_viewTransitions flag in the configuration.

Pre-flight checklist

• I have read the Contributing Guidelines on pull requests.
• If this is a code change: I have written unit tests and/or added dogfooding pages to fully verify the new behavior.
• If this is a new API or substantial change: the PR has an accompanying issue (closes #0000) and the maintainers have approved on my working plan.

Motivation

View Transitions improve SPA navigation UX. Client modules can't implement this because onRouteUpdate fires before the DOM swap. PendingNavigation already keeps the previous route mounted during preload — this PR wraps the final commit in startViewTransition() + flushSync().

#12223

Test Plan

• Unit tests for viewTransitionUtils (8 tests)
• Config validation + snapshot updates
• @docusaurus/core builds successfully
• Manual: navigate between pages in Chrome/Safari 18+
• Manual: verify no animation with prefers-reduced-motion: reduce
• Manual: verify hash-only navigation stays instant
• Manual: verify Firefox fallback (instant, no errors)

Test links

Deploy preview: https://deploy-preview-_____--docusaurus-2.netlify.app/

Related issues/PRs

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