Refactor `MultiDocYaml` loader

What does this MR do and why?

This MR makes several improvements to MultiDocYaml:

1. It refactors MultiDocYaml to use Loader::Yaml to load each document instead of duplicating the logic in Loader::Yaml
2. It updates MultiDocYaml to work for document separators with additional characters in the same line:

   version: 3.0.0
   --- !test/content
   job_x:
     script: ls

3. It updates MultiDocYaml to ignore documents that only contain comments (some existing GitLab maintained CI templates include documents that only have comments):

   version: 3.0.0
   ---
   # This document will be ignored
   ---
   job_x:
     script: ls

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #383375 (closed)