docs: add description about how to get the GCS credential

[flaneur2020]

the credential is essential to get GCS connected, but it's hard for us to memorize how to get the credential from GCS.

it'd be better to have a link about how to get the credential in the docs.

[flaneur2020]

Follow the topic Create a service account key from the Google documentation to create and download a service account key file.
Utilize Base64 encoding to convert the contents of the service account key file into a Base64-encoded string. For example,
base64 -i -o ~/Desktop/base64-encoded-key.txt

[BohuTANG]

There is a description here:
https://docs.databend.com/guides/deploy/deploy/non-production/deploying-databend

[PsiACE]

#772