[MM-46468] Change /help and add /marketplace (#20850)

i18n/en.json

@@ -856,12 +856,16 @@
   },
   {
     "id": "api.command_help.desc",
-    "translation": "Open the Mattermost help page"
+    "translation": "Show Mattermost help message"
   },
   {
     "id": "api.command_help.name",
     "translation": "help"
   },
+  {
+    "id": "api.command_help.success",
+    "translation": "Mattermost is an open source platform for secure communication, collaboration, and orchestration of work across tools and teams.\r\nMattermost contains three key tools:\r\n\r\n**Channels** - Stay connected with your team via 1:1 and group messaging.\r\n**[Playbooks](/playbooks)** - Build and configure repeatable processes to achieve specific and predictable outcomes.\r\n**[Boards](/boards)** - Manage projects and tasks in a Kanban board structure to help your team hit key milestones.\r\n\r\n[View documentation and guides]({{.HelpLink}})"
+  },
   {
     "id": "api.command_invite.channel.app_error",
     "translation": "Error to retrieve the current channel."
@@ -974,6 +978,18 @@
     "id": "api.command_logout.name",
     "translation": "logout"
   },
+  {
+    "id": "api.command_marketplace.desc",
+    "translation": "Open the Marketplace"
+  },
+  {
+    "id": "api.command_marketplace.name",
+    "translation": "marketplace"
+  },
+  {
+    "id": "api.command_marketplace.unsupported.app_error",
+    "translation": "The marketplace command is not supported on your device."
+  },
   {
     "id": "api.command_me.desc",
     "translation": "Do an action"