Enhance NotFoundException documentation

Added XML documentation comments to the `NotFoundException` class and its constructors. This improves code readability and provides clear descriptions for developers using this exception.

EnvelopeGenerator.Application/Exceptions/NotFoundException.cs

@@ -1,11 +1,21 @@
 namespace EnvelopeGenerator.Application.Exceptions;
 
+/// <summary>
+/// Represents an exception that is thrown when a requested resource is not found.
+/// </summary>
 public class NotFoundException : Exception
 {
+    /// <summary>
+    /// Initializes a new instance of the <see cref="NotFoundException"/> class.
+    /// </summary>
     public NotFoundException()
     {
     }
 
+    /// <summary>
+    /// Initializes a new instance of the <see cref="NotFoundException"/> class with a specified error message.
+    /// </summary>
+    /// <param name="message">The message that describes the error.</param>
     public NotFoundException(string? message) : base(message)
     {
     }