Reorganize directories (#3462)

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

docs/examples/emplace.cpp

@@ -0,0 +1,31 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json object = {{"one", 1}, {"two", 2}};
+    json null;
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << null << '\n';
+
+    // add values
+    auto res1 = object.emplace("three", 3);
+    null.emplace("A", "a");
+    null.emplace("B", "b");
+
+    // the following call will not add an object, because there is already
+    // a value stored at key "B"
+    auto res2 = null.emplace("B", "c");
+
+    // print values
+    std::cout << object << '\n';
+    std::cout << *res1.first << " " << std::boolalpha << res1.second << '\n';
+
+    std::cout << null << '\n';
+    std::cout << *res2.first << " " << std::boolalpha << res2.second << '\n';
+}