Add documentation for STORAGE_<LOCATION>_CREDENTIALS

[Nerglej]

Describe the Request

Simply, add some documentation for the STORAGE_<LOCATION>_CREDENTIALS environment variable.

I stumpled upon it when reevaluating if Google Cloud Platform is the correct place to host our service, and then reading https://docs.directus.io/blog/deploying-directus-to-google-cloud-platform-with-docker.html.

I hadn't seen it before, and before seeing this I used a volume to mount a secret file, to use STORAGE_<LOCATION>_KEY_FILENAME.

So I tried using the newly found env variable, but it didn't work, until I stumpled upon another issue here, that had "json:" in front of the value. So I added "json:{...}" and then it worked. Then I realised this is also written under "Environment Syntax Prefix" in the config options article, so that was just an oopsie.

I also don't know if this is GCP specific or if it's generally available for all storage providers. Nonetheless the variable should be documented somewhere.

Maintenance Strategy

This should not require any maintanence once it's properbly written by someone who knows more about the subject.