Reorganize directories (#3462)

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

docs/examples/swap__string_t.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON value
+    json value = { "the good", "the bad", "the ugly" };
+
+    // create string_t
+    json::string_t string = "the fast";
+
+    // swap the object stored in the JSON value
+    value[1].swap(string);
+
+    // output the values
+    std::cout << "value = " << value << '\n';
+    std::cout << "string = " << string << '\n';
+}