Improve Unstable Cache Docs (#65942)
Unstable Cache has always had a very lacking documentation page, even
though it adding its own new feature ``key parts`` . Its a normal
occurrence for us in the NextJS Discord to see new people and more
advanced users question what is the use of key parts, since the example
in the docs. was very lacking.
I have made a few changes, like adding a much more functional example
which showcases all the features of unstable cache, and also a small
difference section between tags and key parts.
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Co-authored-by: Sam Ko <sam@vercel.com>
