GCP documentation page in Fedora CoreOS docs

[dustymabe]

Now that we are uploading images to GCP we need to add a documentation page dedicated to GCP in our FCOS docs (smilar to the one we have for Azure).

Rough notes to get started:

gcloud auth activate-service-account --key-file=/root/creds-gcp.json
gcloud compute images describe-from-family --project fedora-coreos-cloud fedora-coreos-stable
gcloud compute instances create node1 --project fedora-coreos-testing --zone us-east1-b --image-project fedora-coreos-cloud --image-family fedora-coreos-stable --metadata-from-file=user-data=/root/user-systemd.ign

[dustymabe]

Thanks @lucab for volunteering to make this docs page for us.

[lucab]

Docs PR at coreos/fedora-coreos-docs#70.
Any native-speaking proofreader is welcome!