Add serve_header.py for rapid testing on Compiler Explorer (#3456)

tools/serve_header/serve_header.yml.example

@@ -0,0 +1,15 @@
+# all paths are relative to the project root
+
+# the root directory for the web server
+# root: .
+
+# configure SSL
+# https:
+#   enabled: true
+# these filenames are listed in .gitignore
+#  cert_file: localhost.pem
+#  key_file: localhost-key.pem
+
+# address and port for the server to listen on
+# bind: null
+# port: 8443