docs: updates to Cache Components Caching (#95452)
- Questions kept coming up about where `use cache` results actually
live, and how static shells, App Shells, and ISR relate
- the answers were spread across several pages, but not tied together on
the getting-started guide
- The runtime prefetching example also implied cookie data needs
`allow-runtime`, when the App Shell already carries session data
- `allow-runtime` is really about URL data.
**Changes** (`docs/app/getting-started/caching`)
- Reworked the Usage intro to lead with the cache lifetime concept and
hand off to Prerendering
- Added a **Where cached content is stored** section: server cache,
remote handler, and browser, framed around a cached result being a
serialized payload that travels to each.
- > if I had time to make it shorter, I would...
- We will most likely have a visual aid there and trim it down
- Added a static shell ↔ App Shell explanation in Prerendering, and a
short Incremental Static Regeneration pointer to the ISR guide
- Corrected the runtime prefetching section to the URL-data model
(`searchParams`/`params`)
- replaced the cookie example with a `/search` one that actually needs
`allow-runtime`.
- `catchError` (stabilized).
- Cross-linked `stale`/`revalidate`/`expire` to `cacheLife`
- reviewed and fixed the code-block highlights.
