Adds new gitlab-sshd page to left nav

Add navigation item to global navigation

• Link to the merge request that introduced the new page: gitlab!111383 (merged) created it, and gitlab!111505 (merged) moved it to its new home.
• Give the MR a descriptive title.
• Optional. Link to the review app showing the new page:

Not yet visible, but it'll be at https://docs.gitlab.com/ee/administration/operations/gitlab_sshd.html after the next docs deployment.

Related links

• Related to Update docs on gitlab-sshd (gitlab!111383 - merged) which initially provided the information
• Related to Split gitlab-sshd docs into its own document (gitlab!111505 - merged) which carved off the page
• Related to Implement gitlab-sshd on GitLab SaaS (gitlab-com/gl-infra&603 - closed) the epic for the engineering work
• Related to Add gitlab-sshd configuration support (omnibus-gitlab!6446 - merged) where Omnibus support was added