Dynamically generate extras pages instead of hardcoded list

This means that any future additions into `guides` will automatically be
built by ExDoc when documentation is generated. There is no longer a
need to add the page and then update the Mix file with the new page.

mix.exs

@@ -70,23 +70,7 @@ defmodule Nostrum.Mixfile do
   end
 
   def extras do
-    [
-      "guides/intro/intro.md",
-      "guides/intro/api.md",
-      "guides/intro/application_commands.md",
-      "guides/intro/gateway_intents.md",
-      "guides/functionality/state.md",
-      "guides/functionality/event_handling.md",
-      "guides/functionality/voice.md",
-      "guides/advanced/pluggable_caching.md",
-      "guides/advanced/multi_node.md",
-      "guides/advanced/hot_code_upgrade.md",
-      "guides/advanced/gateway_compression.md",
-      "guides/advanced/manual_sharding.md",
-      "guides/cheat-sheets/api.cheatmd",
-      "guides/cheat-sheets/qlc.cheatmd",
-      "guides/cheat-sheets/voice.cheatmd"
-    ]
+    Path.wildcard("guides/**/*.md")
   end
 
   def groups_for_modules do