Truncates gitlab-sshd page

Truncates the page and adds a link pointing to the GitLab repo.

doc/gitlab-sshd.md

----
-stage: Create
-group: Source Code
-info: To determine the technical writer assigned to the Stage/Group associated with this page, see https://about.gitlab.com/handbook/engineering/ux/technical-writing/#assignments
----
-
 # gitlab-sshd
 
-`gitlab-sshd` is a binary in [`gitlab-shell`](https://gitlab.com/gitlab-org/gitlab-shell)
-which runs as a persistent SSH daemon. It will replace `OpenSSH` on GitLab SaaS,
-and eventually other cloud-native environments. Instead of running an `sshd` process,
-we run a `gitlab-sshd` process that does the same job, in a more focused manner:
-
-```mermaid
-sequenceDiagram
-    participant Git on client
-    participant GitLab SSHD
-    participant Rails
-    participant Gitaly
-    participant Git on server
-
-    Note left of Git on client: git fetch
-    Git on client->>+GitLab SSHD: ssh git fetch-pack request
-    GitLab SSHD->>+Rails: GET /internal/api/authorized_keys?key=AAAA...
-    Note right of Rails: Lookup key ID
-    Rails-->>-GitLab SSHD: 200 OK, command="gitlab-shell upload-pack key_id=1"
-    GitLab SSHD->>+Rails: GET /internal/api/allowed?action=upload_pack&key_id=1
-    Note right of Rails: Auth check
-    Rails-->>-GitLab SSHD: 200 OK, { gitaly: ... }
-    GitLab SSHD->>+Gitaly: SSHService.SSHUploadPack request
-    Gitaly->>+Git on server: git upload-pack request
-    Note over Git on client,Git on server: Bidirectional communication between Git client and server
-    Git on server-->>-Gitaly: git upload-pack response
-    Gitaly -->>-GitLab SSHD: SSHService.SSHUploadPack response
-    GitLab SSHD-->>-Git on client: ssh git fetch-pack response
-```
+This page
+[has moved into the `gitlab` repository](https://docs.gitlab.com/ee/development/gitlab_shell/gitlab_sshd.html).