Reorganize directories (#3462)

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

docs/examples/get_ref.cpp

@@ -0,0 +1,27 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON number
+    json value = 17;
+
+    // explicitly getting references
+    auto r1 = value.get_ref<const json::number_integer_t&>();
+    auto r2 = value.get_ref<json::number_integer_t&>();
+
+    // print the values
+    std::cout << r1 << ' ' << r2 << '\n';
+
+    // incompatible type throws exception
+    try
+    {
+        auto r3 = value.get_ref<json::number_float_t&>();
+    }
+    catch (json::type_error& ex)
+    {
+        std::cout << ex.what() << '\n';
+    }
+}