docs: add link to IAM setup guide in Prerequisites

[akaraj187]

Resolves #50 Added a direct link to the Google Cloud documentation for setting up IAM authentication in the Prerequisites section. This helps users who are new to Cloud SQL understand how to configure the required roles.

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[averikitsch]

Thanks for this update. Can we also add how to create an IAM Cloud SQL user? If they don't set username/password it should default to the local IAM user.

[akaraj187]

Done! I added the link for creating IAM users and noted the default credential behavior. Ready for review.

[akaraj187]

I noticed the License Header Check is failing on multiple .yml files that I didn't touch.
Is there anything I should do ,or is this a known issue?

[averikitsch]

I noticed the License Header Check is failing on multiple .yml files that I didn't touch. Is there anything I should do ,or is this a known issue?

This is fixed in another PR. Thanks!