docs: cover data apps in the caching reference

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Adds data apps to the caching reference page so it stays in sync with the existing data apps guide.

• Adds a Data apps bullet to the "use caching" list explaining that every metric query runs through the async query service, that the Refresh button invalidates the app's cache, and that lots of interactive filter controls produce more distinct cache entries than an equivalent dashboard.
• Adds an External connections in data apps bullet to the "DO NOT use caching" list, calling out that third-party HTTP fetches through the Lightdash proxy are not covered by results caching.
• Cross-links /guides/developer/caching from the "Refreshing the preview" section of the data apps guide.

Existing structure and tone are preserved.

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