Update maven.gitlab-ci.yml feature flag checking for consistent behaviour with other CI templates

The script to retrieve GitLab access tokens in ci-templates / auto-devops / maven.gitlab-ci.yml currently checks for "1" to indicate that a feature is disabled.

For consistency with variable usage in other common templates, the script should use any value to indicate the flag is set.

Update the Guidebook page for this template to explicitly document that setting a value indicates the feature is disabled, the absence of setting one of the feature flag variables indicates the feature is enabled.