Add initial implementation of PlunkEmailSender and update .gitignore.

- Removed `appsettings.Development.json` and excluded it from version control.
- Updated `API.csproj` to remove unnecessary folder reference.

API/Services/PlunkEmailSender.cs

@@ -0,0 +1,11 @@
+using Microsoft.AspNetCore.Identity.UI.Services;
+
+namespace API.Services;
+
+public class PlunkEmailSender : IEmailSender
+{
+    public Task SendEmailAsync(string email, string subject, string htmlMessage)
+    {
+        throw new NotImplementedException();
+    }
+}