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.output

@@ -0,0 +1,22 @@
+{"one":1,"two":2}
+
+[1,2,4,8,16]
+
+{
+    "one": 1,
+    "two": 2
+}
+
+[
+  1,
+  2,
+  4,
+  8,
+  16
+]
+
+{
+	"one": 1,
+	"two": 2
+}
+