Add docs for new autodevops variables

What does this MR do and why?

This is a small documentation change to describe these two already merged MR functions:

• gitlab-org/cluster-integration/auto-deploy-image!355 (merged)
• gitlab-org/cluster-integration/auto-deploy-image!356 (merged)

Add the following auto-deploy config description to documentation

• AUTO_DEVOPS_CHART_CUSTOM_ONLY
• AUTO_DEVOPS_CHART_REPOSITORY_INSECURE
• AUTO_DEVOPS_CHART_VERSION