Reorganize directories (#3462)

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

docs/examples/basic_json__copyassignment.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json a = 23;
+    json b = 42;
+
+    // copy-assign a to b
+    b = a;
+
+    // serialize the JSON arrays
+    std::cout << a << '\n';
+    std::cout << b << '\n';
+}