docs: Add example about retriever

[devalexandre]

A example about retriever, how use and why need use

PR Checklist

• Read the Contributing documentation.
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• Name your Pull Request title clearly, concisely, and prefixed with the name of the primarily affected package you changed according to Good commit messages (such as memory: add interfaces for X, Y or util: add whizzbang helpers).
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[tmc]

Thanks for adding this, a small comment that i think would improve readability/interpretability.

[tmc]

I'm happy to accept this change but will hold off for a bit so you have an opportunity to add a bit more detail as requested

[tmc]

LGTM