Clean up documentation for Airshipctl & CAPG integration

[eak13]

Problem description
As changes have occurred in the Airship deployment process as it relates to GCP, the documentation has not been kept up to date.
https://docs.airshipit.org/airshipctl/providers/cluster_api_gcp.html

Tasks:

• Review the CAPG documentation while working through the deployment process
• Make any updates to the documentation md
• Push the update to the website (which will happen automatically upon merge).

[DannyMassa]

Assign to me plz.