Add short_description to mastodon v1 instance api response.

Some third party client applications are expecting the
short_description key in the v1 instance response.

See this issue:
https://github.com/Dimillian/IceCubesApp/issues/16
and this issue
https://github.com/Dimillian/IceCubesApp/issues/2105#issuecomment-2231466416

short_description is a required field as per the masto
api docs.
https://docs.joinmastodon.org/entities/V1_Instance/#short_description

lib/pleroma/web/mastodon_api/views/instance_view.ex

@@ -17,6 +17,7 @@ def render("show.json", _) do
       uri: Pleroma.Web.Endpoint.url(),
       title: Keyword.get(instance, :name),
       description: Keyword.get(instance, :description),
+      short_description: Keyword.get(instance, :description),
       version: "#{@mastodon_api_level} (compatible; #{Pleroma.Application.named_version()})",
       email: Keyword.get(instance, :email),
       urls: %{

test/pleroma/web/mastodon_api/controllers/instance_controller_test.exs

@@ -23,6 +23,7 @@ test "get instance information", %{conn: conn} do
              "uri" => _,
              "title" => _,
              "description" => _,
+             "short_description" => _,
              "version" => _,
              "email" => from_config_email,
              "urls" => %{