Add specific documentation around user management. Closes #1978

- Briefly cover sshcommand internals
- Show several examples for using sshcommand
- Split out docs onto a new page and relink to it where necessary
- Refer to user-auth plugin trigger for scoping commands to specific users.

[ci skip]

docs/template.html

@@ -117,6 +117,7 @@
             <a href="/dokku/remote-commands/" class="list-group-item">Remote Commands</a>
             <a href="/dokku/deployment/one-off-processes/" class="list-group-item">One Off Processes/Cron</a>
             <a href="/dokku/process-management/" class="list-group-item">Scaling Apps</a>
+            <a href="/dokku/deployment/user-management/" class="list-group-item">User Management</a>
             <a href="/dokku/checks-examples/" class="list-group-item">Zero Downtime Deploy Checks</a>
 
             <a href="#" class="list-group-item disabled">Configuration</a>