Implement `-domain-config-source=auto`

Follow-up issue on enabling different source configuration for Pages based on #210010 (comment 342953588) and #217912 (comment 344034359)

1. Implement -domain-config-source=auto
2. Update documentation on -domain-config-source=auto usage

When auto is set, GitLab Pages will use any of the available sources when ready (either disk or gitlab). The gitlab source is the preferred one over disk. disk will be deprecated and removed soon.