Reorganize directories (#3462)

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

docs/examples/to_string.cpp

@@ -0,0 +1,20 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+using std::to_string;
+
+int main()
+{
+    // create values
+    json j = {{"one", 1}, {"two", 2}};
+    int i = 42;
+
+    // use ADL to select best to_string function
+    auto j_str = to_string(j);  // calling nlohmann::to_string
+    auto i_str = to_string(i);  // calling std::to_string
+
+    // serialize without indentation
+    std::cout << j_str << "\n\n"
+              << i_str << std::endl;
+}