docs: document preloading with Cache Components

[aurorascharff]

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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