Reorganize directories (#3462)

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

docs/examples/insert__count.cpp

@@ -0,0 +1,17 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create a JSON array
+    json v = {1, 2, 3, 4};
+
+    // insert number 7 copies of number 7 before number 3
+    auto new_pos = v.insert(v.begin() + 2, 7, 7);
+
+    // output new array and result of insert call
+    std::cout << *new_pos << '\n';
+    std::cout << v << '\n';
+}