Add spdx copyright and license to remaining files

.github/workflows/dco-welcome.yml

@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: 2026 Fabian Ruffy
+#
+# SPDX-License-Identifier: Apache-2.0
+
 name: DCO Assistant for First-Time Contributors
 
 on:

.github/workflows/test-exercises.yml

@@ -1,3 +1,7 @@
+# SPDX-FileCopyrightText: 2026 An Nguyen
+#
+# SPDX-License-Identifier: Apache-2.0
+
 name: P4 Tutorials CI
 
 on:

.gitignore

@@ -1,3 +1,5 @@
+# SPDX-FileCopyrightText: 2016 Antonin Bas
+#
 # SPDX-License-Identifier: Apache-2.0
 
 # Python byte code

CONTRIBUTING.md

@@ -1,5 +1,9 @@
+<!--
+SPDX-FileCopyrightText: 2024 Abhinav Kumar
+
+SPDX-License-Identifier: Apache-2.0
+-->
 
-[comment]: # (SPDX-License-Identifier:  Apache-2.0)
 ---
 
 Thank you for your interest in contributing to the P4 Compiler tutorials repository! Your contributions are important and will help to improve the project for everyone. Before you begin, please consider the guidelines below.