Add ViewTransition and addTransitionType to the reference indexes (#8673)

Both became stable in React 19.3 and both have reference pages and
sidebar entries, but neither is listed on its overview page:

- `<ViewTransition>` is missing from Built-in React Components, which
  already lists its 19.3 sibling `<Activity>`.
- `addTransitionType` is missing from Built-in React APIs, which
  already lists `cache`, `cacheSignal` and `captureOwnerStack`.

CONTRIBUTING.md says the API Reference "is intended to be exhaustive",
so a reader browsing either overview page currently can't discover
either API.

Co-authored-by: Claude Opus 5 (1M context) <noreply@anthropic.com>

Author: BIGSUS24

src/content/reference/react/apis.md

@@ -14,6 +14,7 @@ In addition to [Hooks](/reference/react/hooks) and [Components](/reference/react
 * [`lazy`](/reference/react/lazy) lets you defer loading a component's code until it's rendered for the first time.
 * [`memo`](/reference/react/memo) lets your component skip re-renders with same props. Used with [`useMemo`](/reference/react/useMemo) and [`useCallback`.](/reference/react/useCallback)
 * [`startTransition`](/reference/react/startTransition) lets you mark a state update as non-urgent. Similar to [`useTransition`.](/reference/react/useTransition)
+* [`addTransitionType`](/reference/react/addTransitionType) lets you specify the cause of a Transition. Used with [`startTransition`](/reference/react/startTransition) and [`<ViewTransition>`.](/reference/react/ViewTransition)
 * [`act`](/reference/react/act) lets you wrap renders and interactions in tests to ensure updates have processed before making assertions.
 * [`cache`](/reference/react/cache) lets you cache the result of a data fetch or computation.
 * [`cacheSignal`](/reference/react/cacheSignal) lets you know when the `cache()` lifetime is over.

src/content/reference/react/components.md

@@ -17,6 +17,7 @@ React exposes a few built-in components that you can use in your JSX.
 * [`<Suspense>`](/reference/react/Suspense) lets you display a fallback while the child components are loading.
 * [`<StrictMode>`](/reference/react/StrictMode) enables extra development-only checks that help you find bugs early.
 * [`<Activity>`](/reference/react/Activity) lets you hide and restore the UI and internal state of its children.
+* [`<ViewTransition>`](/reference/react/ViewTransition) lets you animate a component tree with Transitions and Suspense.
 
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