Amalgamate the forward declaration header (#3679)

* Amalgamate the forward declaration header

* Update CONTRIBUTING and PULL_REQUEST_TEMPLATE

* Update documentation

tools/amalgamate/config_json_fwd.json

@@ -0,0 +1,8 @@
+{
+	"project": "JSON for Modern C++",
+	"target": "single_include/nlohmann/json_fwd.hpp",
+	"sources": [
+		"include/nlohmann/json_fwd.hpp"
+	],
+	"include_paths": ["include"]
+}