Add default config for masto_fe

Related to: https://git.pleroma.social/pleroma/mastofe/merge_requests/22

config/config.exs

@@ -215,6 +215,9 @@ config :pleroma, :frontend_configurations,
     scopeCopy: true,
     subjectLineBehavior: "email",
     alwaysShowSubjectInput: true
+  },
+  masto_fe: %{
+    showInstanceSpecificPanel: true
   }
 
 config :pleroma, :activitypub,

docs/config.md

@@ -129,7 +129,7 @@ See: [logger’s documentation](https://hexdocs.pm/logger/Logger.html) and [ex_s
 
 ## :frontend_configurations
 
-This can be used to configure a keyword list that keeps the configuration data for any kind of frontend. By default, settings for `pleroma_fe` are configured.
+This can be used to configure a keyword list that keeps the configuration data for any kind of frontend. By default, settings for `pleroma_fe` and `masto_fe` are configured.
 
 Frontends can access these settings at `/api/pleroma/frontend_configurations`