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/api/basic_json/accept.md

@@ -96,7 +96,7 @@ Linear in the length of the input. The parser is a predictive LL(1) parser.
 ## See also
 
 - [parse](parse.md) - deserialize from a compatible input
-- [operator>>](operator_gtgt.md) - deserialize from stream
+- [operator>>](../operator_gtgt.md) - deserialize from stream
 
 ## Version history