Reorganize directories (#3462)

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

tests/cmake_fetch_content2/project/main.cpp

@@ -0,0 +1,8 @@
+#include <nlohmann/json.hpp>
+
+int main(int argc, char **argv)
+{
+    nlohmann::json j;
+
+    return 0;
+}