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/mkdocs/docs/features/parsing/json_lines.md

@@ -36,7 +36,7 @@ JSON Lines input with more than one value is treated as invalid JSON by the [`pa
 
 !!! warning "Note"
 
-    Using [`operator>>`](../../api/basic_json/operator_gtgt.md) like
+    Using [`operator>>`](../../api/operator_gtgt.md) like
     
     ```cpp
     json j;