Reorganize directories (#3462)

* 🚚 move files
* 🚚 rename doc folder to docs
* 🚚 rename test folder to tests

tests/cuda_example/json_cuda.cu

@@ -0,0 +1,11 @@
+#include <nlohmann/json.hpp>
+
+int main()
+{
+    nlohmann::ordered_json json = {"Test"};
+    json.dump();
+
+    // regression for #3013 (ordered_json::reset() compile error with nvcc)
+    nlohmann::ordered_json metadata;
+    metadata.erase("key");
+}