Reorganize directories (#3462)

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

docs/examples/count.cpp

@@ -0,0 +1,18 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON object
+    json j_object = {{"one", 1}, {"two", 2}};
+
+    // call count()
+    auto count_two = j_object.count("two");
+    auto count_three = j_object.count("three");
+
+    // print values
+    std::cout << "number of elements with key \"two\": " << count_two << '\n';
+    std::cout << "number of elements with key \"three\": " << count_three << '\n';
+}