Add operator<<(json_pointer) (#3601)

* Add operator<< for json_pointer

* Deprecate json_pointer::operator string_t()

* Update documentation

* Move operator<<(basic_json) example

* Add example

* Add mkdocs-redirects

* Move operator<< and operator>> doc pages out of basic_json/

* Rename JSON pointer operator_string to operator_string_t

* Add unit test

docs/examples/operator_ltlt__basic_json.cpp

@@ -0,0 +1,21 @@
+#include <iostream>
+#include <iomanip>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // create JSON values
+    json j_object = {{"one", 1}, {"two", 2}};
+    json j_array = {1, 2, 4, 8, 16};
+
+    // serialize without indentation
+    std::cout << j_object << "\n\n";
+    std::cout << j_array << "\n\n";
+
+    // serialize with indentation
+    std::cout << std::setw(4) << j_object << "\n\n";
+    std::cout << std::setw(2) << j_array << "\n\n";
+    std::cout << std::setw(1) << std::setfill('\t') << j_object << "\n\n";
+}