docs: document preloading with Cache Components (#97864)
## Summary
- add the preload pattern to the current App Router data fetching guide
- distinguish automatic fetch memoization, React cache for non-fetch
data, and Cache Functions
- clarify request-scoped private caching and when stale: Infinity should
not constrain the route stale time
The previous-model caching guide remains unchanged.
## Verification
- Prettier on both edited docs
- alex on both edited docs
- git diff --check
- Not run: full framework build because this is a documentation-only
change
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