Medusa System Added

Read about the medusa system [here](https://nadekobot.readthedocs.io/en/latest/medusa/creating-a-medusa/)

src/NadekoBot/Common/ModuleBehaviors/IInputTransformer.cs

@@ -1,8 +1,23 @@
 namespace NadekoBot.Common.ModuleBehaviors;
 
+/// <summary>
+/// Implemented by services which may transform input before a command is searched for
+/// </summary>
 public interface IInputTransformer
 {
-    Task<string> TransformInput(
+    /// <summary>
+    /// Ran after a non-bot message was received
+    /// <see cref="IExecOnMessage"/> ->
+    /// *<see cref="IInputTransformer"/>* ->
+    /// <see cref="IExecPreCommand"/> ->
+    /// [<see cref="IExecPostCommand"/> OR <see cref="IExecNoCommand"/>]
+    /// </summary>
+    /// <param name="guild">Guild</param>
+    /// <param name="channel">Channel in which the message was sent</param>
+    /// <param name="user">User who sent the message</param>
+    /// <param name="input">Content of the message</param>
+    /// <returns>New input, if any, otherwise null</returns>
+    Task<string?> TransformInput(
         IGuild guild,
         IMessageChannel channel,
         IUser user,