Add Html.render_doc/1

Wraps HtmlDocComponent.render_doc/1 and converts the resulting struct
to binary.

lib/outlook/internal_tree/html.ex

@@ -74,4 +74,11 @@ defmodule Outlook.InternalTree.Html do
   end
 
   def to_html_preview([], _target_id), do: ""
+
+
+  def render_doc(tree) do
+    OutlookWeb.HtmlDocComponent.render_doc(%{tree: tree})
+    |> Phoenix.HTML.Safe.to_iodata()
+    |> IO.iodata_to_binary()
+  end
 end