howto/gitlab: update GitHub mirroring instructions

GitHub now supports deploy keys with write permissions, so there is no
need anymore for a surrogate GitHub account. Using the deploy key allows
for a much simplified setup process, in addition to making each key
project-specific on the GitHub side.

howto/gitlab.md

@@ -174,8 +174,8 @@ new mirror from GitLab. In this example, we're going to mirror the
     [settings/repository](https://gitlab.torproject.org/tpo/web/manual/-/settings/repository) part of the project
 
  2. as a Git repository URL, enter:
- 
-        ssh://git@github.com/torproject/tb-manual.git
+
+        ssh://git@github.com/torproject/manual.git
 
  3. click "detect host keys"
 
@@ -187,31 +187,24 @@ new mirror from GitLab. In this example, we're going to mirror the
     repositories". click the little "paperclip" icon which says "Copy
     SSH public key"
 
- 7. head over to https://github.com and authenticate with
-    `torproject-pusher`, password is in `external-services-git`, in
-    the password manager
-
- 8. go to [settings/keys](https://github.com/settings/keys) and hit [new SSH key](https://github.com/settings/ssh/new)
+ 7. head over to the [settings/keys](https://github.com/torproject/manual/settings/keys)
+    section of the target GitHub project and click "Add deploy key"
 
- 9. paste the public key in the bigger field, as a title, use the URL
-    of the repository, for example:
-    
         Title: https://gitlab.torproject.org/tpo/web/manual mirror key
-        Key: ssh-rsa AAAA[...]
-
- 10. click "Add SSH key"
+        Key: <paste public key here>
 
- 11. to speed up the process, you can [import the repository in
-     GitHub](https://github.com/new/import), otherwise create a [new repository](https://github.com/new). in *both*
-     cases make sure you change the namespace from the default
-     (`torproject-pusher`, which is incorrect) to the `torproject`
-     namespace (which is correct)
+ 8. check the "Allow write access" checkbox and click "Add key"
 
- 12. then hit the "reload" button in the repositories mirror list
+ 9. back in the "Mirroring repositories" section of the GitLab project, click
+    the "Update now" button represented by circling arrows
 
 If there is an error, it will show up as a little red "Error"
 button. Hovering your mouse over the button will show you the error.
 
+If you want retry the "Update now" button, you need to let the update interval
+pass (1 minute for protected branch mirroring, 5 minutes for all branches)
+otherwise it will have no effect.
+
 ## How to find the right emoji?
 
 It's possible to add "reaction emojis" to comments and issues and