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.

.gitignore

@@ -90,4 +90,7 @@ wwwroot/
 # SQLite Datenbank-Dateien
 *.db
 *.db-shm
-*.db-wal
+*.db-wal
+
+# AppSettings Development files
+appsettings.Development.json