Add incremental delivery guide to documentation extras

Include guides/incremental-delivery.md in the mix.exs extras list so it
appears in the generated documentation alongside other guides.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: jwaldrip

mix.exs

@@ -115,6 +115,7 @@ defmodule Absinthe.Mixfile do
       "guides/dataloader.md",
       "guides/context-and-authentication.md",
       "guides/subscriptions.md",
+      "guides/incremental-delivery.md",
       "guides/custom-scalars.md",
       "guides/importing-types.md",
       "guides/importing-fields.md",