Modernize docs
Modernize docs

This change catches up with spring-cloud-build to modernize the docs. 
It mostly removes things we didn't need from the docs/pom.xml file (and adds one Maven resource plugin that Asciidoctor needs).
spring-cloud/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,spring-cloud/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,GoogleCloudPlatform/spring-cloud-gcp,spring-cloud/spring-cloud-gcp,spring-cloud/spring-cloud-gcp