docs: update a broken link of gemini cli

[Marukome0743]

Summary

Documentation:

• Update the README to point the Gemini CLI reference link to the correct configuration documentation path.

Before	After
https://github.com/google-gemini/gemini-cli/blob/main/docs/cli/configuration.md	https://github.com/google-gemini/gemini-cli/blob/main/docs/get-started/configuration.md

Ref: docs: Remove redundant and duplicate documentation files #14699