Docs: Add Silver badge to group level project templates

Problem to solve

Currently, our documentation for custom group-level project templates only displays the badge for Premium when it is also available on GitLab.com Silver.

Proposal

The Silver badge should be added to this documentation to alleviate any confusion from GitLab.com Silver users as to whether or not they can use this feature on the free plan.

We were previously inadvertently allowing Free users access to this feature as outlined in this issue but that has since been resolved.