[Enhancement] Capture playlist_index for use with output templates (#315)

* Started capturing playlist_index on indexing pass

* Added playlist_index as a media item field

* Added playlist index to output variable templates

* Improved the way playlist_indexes are rejected on update

* Updated docs

* Undid unneeded changes

priv/repo/migrations/20240715212133_add_playlist_index_to_media_items.exs

@@ -0,0 +1,9 @@
+defmodule Pinchflat.Repo.Migrations.AddPlaylistIndexToMediaItems do
+  use Ecto.Migration
+
+  def change do
+    alter table(:media_items) do
+      add :playlist_index, :integer, null: false, default: 0
+    end
+  end
+end