Reorganize directories (#3462)

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

docs/examples/json_pointer__pop_back.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create empty JSON Pointer
+    json::json_pointer ptr("/foo/bar/baz");
+    std::cout << ptr << '\n';
+
+    // call pop_back()
+    ptr.pop_back();
+    std::cout << ptr << '\n';
+
+    ptr.pop_back();
+    std::cout << ptr << '\n';
+
+    ptr.pop_back();
+    std::cout << ptr << '\n';
+}