feat: allow generating an ssh deploy key via the git plugin

While this might not seem super intuitive, the key is mostly related to git management, not the ssh-keys we currently use to add/remove access to push to dokku.

plugins/git/help-functions

@@ -34,6 +34,7 @@ fn-help-content() {
     git:load-image <app> <docker-image> [<git-username> <git-email>], Updates an app's git repository with a docker image loaded from stdin
     git:sync [--build] <app> <repository> [<git-ref>], Clone or fetch an app from remote git repo
     git:initialize <app>, Initialize a git repository for an app
+    git:generate-deploy-key, Generates a deploy ssh key
     git:public-key, Outputs the dokku public deploy key
     git:report [<app>] [<flag>], Displays a git report for one or more apps
     git:set <app> <property> (<value>), Set or clear a git property for an app