docs: add cache base option example & description

[LouisHaftmann]

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Adds a description for the cache base option in route rules and an example on how to use a specific storage mountpoint for a group of routes.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.

[codecov]

Codecov Report

Merging #781 (ab81a39) into main (4ed8efe) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main     #781   +/-   ##
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  Coverage   70.07%   70.07%           
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  Files          57       57           
  Lines        5229     5229           
  Branches      575      575           
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  Hits         3664     3664           
  Misses       1556     1556           
  Partials        9        9           

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[pi0]

Thanks!