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/json_pointer__operator_string_t.cpp

@@ -0,0 +1,19 @@
+#include <iostream>
+#include <nlohmann/json.hpp>
+
+using json = nlohmann::json;
+
+int main()
+{
+    // different JSON Pointers
+    json::json_pointer ptr1("/foo/0");
+    json::json_pointer ptr2("/a~1b");
+
+    // implicit conversion to string
+    std::string s;
+    s += ptr1;
+    s += "\n";
+    s += ptr2;
+
+    std::cout << s << std::endl;
+}