docs(adopting-partial-prefetching): reframe intro around use cache pages
Pre-PPF behavior was nuanced (static routes prefetched in full, dynamic
routes prefetched nothing beyond loading.tsx). The behavior change PPF
introduces is specifically about Cache Components pages: a <Link> to a
'use cache' page no longer prefetches the cached page content by
default — only the App Shell. The cached content ships per-link when
prefetch={true} is set.
Rewrite the intro and Before/After table to lead with that framing
instead of the misleading 'every page is prefetched in full today'
premise.