docs: make purge-cache section more precise and self-explanatory

[krystiannowak]

Description

Improve clarity of the purge-cache README section to make the cache layer targeting more self-explanatory and coherent with the boilerplate and official documentation.

Changes

• Added ASCII diagram showing which cache layer purge-cache targets
• Clarified that the command purges both fetch cache entries (from fetch() calls) and Core Cache API entries
• Added explicit note explaining the Surrogate-Key header context difference between inner (origin/Core Cache keys) and outer (Edge Function response headers) cache layers
• Added link to CDN Cache Purge API for outer cache purging

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)