Add GitLab Runner Helm Chart documenation for custom certificates

What does this MR do?

This outlines how to provide the custom ssl certificate to the runner for accessing GitLab in the case that GitLab is using a custom/self-signed certificate

Why was this MR needed?

This depends on https://gitlab.com/charts/charts.gitlab.io/merge_requests/44 being merged and released.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/charts/charts.gitlab.io/issues/43