chore(docs): improve GCP docs (#2318)

docs/tutorials/gcp/authentication.md

@@ -0,0 +1,29 @@
+# GCP authentication
+
+Prowler will use by default your User Account credentials, you can configure it using:
+
+- `gcloud init` to use a new account
+- `gcloud config set account <account>` to use an existing account
+
+Then, obtain your access credentials using: `gcloud auth application-default login`
+
+Otherwise, you can generate and download Service Account keys in JSON format (refer to https://cloud.google.com/iam/docs/creating-managing-service-account-keys) and provide the location of the file with the following argument:
+
+```console
+prowler gcp --credentials-file path
+```
+
+> `prowler` will scan the GCP project associated with the credentials.
+
+
+Prowler will follow the same credentials search as [Google authentication libraries](https://cloud.google.com/docs/authentication/application-default-credentials#search_order):
+
+1. [GOOGLE_APPLICATION_CREDENTIALS environment variable](https://cloud.google.com/docs/authentication/application-default-credentials#GAC)
+2. [User credentials set up by using the Google Cloud CLI](https://cloud.google.com/docs/authentication/application-default-credentials#personal)
+3. [The attached service account, returned by the metadata server](https://cloud.google.com/docs/authentication/application-default-credentials#attached-sa)
+
+Those credentials must be associated to a user or service account with proper permissions to do all checks. To make sure, add the following roles to the member associated with the credentials:
+
+  - Viewer
+  - Security Reviewer
+  - Stackdriver Account Viewer