Add new Gitaly TLS support topic to navigation

What does this MR do and why?

As we start to pull apart the Gitaly configuration documentation (for example: gitlab!137613 (merged)), we'll need to add navigation items for them.

Merge request 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 merge request.