doc: Complete copyright and licenses

- Point out licensing terms in docs.
- Add missing SPDX tags
- Update the SPDX checker to check all the files we want to check.
- Include spdx-ensure in CI.

hw/application_fpga/tools/makehex.py

@@ -1,5 +1,8 @@
 #!/usr/bin/env python3
 #
+# SPDX-FileCopyrightText: Claire Xenia Wolf <claire@yosyshq.com>
+# SPDX-License-Identifier: CC0-1.0
+
 # This is free and unencumbered software released into the public domain.
 #
 # Anyone is free to copy, modify, publish, use, compile, sell, or