Add documentation for GCR private registry

[AgustinRamiroDiaz]

Thanks for the new documentation in https://www.okteto.com/docs/self-hosted/install/custom-resource-definitions/#private-registries

I had some problems while trying to figure out how to set up the access for GCR.
I managed to configure it through:

1. Creation of GCP Service Account with permissions to pull/push to the gcr.io registry
2. Created a key for that GCP Service Account
3. Added to the Private Registry configuration in the okteto settings UI
   • Hostname: gcr.io
   • Username: _json_key
   • Password: the raw json string from the key created in step 2

[abhayguptas]

Can I do this?