docs: add information about index storage in cache folder

[stephantul]

Simple PR little doc update.

I added a special section for the configuration of the storage.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[greptile-apps]

Confidence Score: 5/5

Documentation-only change; no executable code is touched and the HTML structure of the README renders correctly.

The change is purely structural reorganization of existing README content — moving a paragraph into its own collapsible section. The

Details

tags are balanced in the final file and the content is accurate.

No files require special attention.

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