Update Google Provider documentation

[eak13]

Enhancement description
The Google Cluster API Provider documentation needs to be brought up to date to match the latest Airshipctl processing. This would include the use of phase plans and any other enhancements that have been implemented.

https://github.com/airshipit/airshipctl/blob/master/docs/source/providers/cluster_api_gcp.md

Please note this work should be coordinated with @mf4716 so it can be developed in the correct repository.