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/requirements.txt

@@ -25,6 +25,7 @@ mkdocs-git-revision-date-localized-plugin==1.0.1
 mkdocs-material==8.2.10
 mkdocs-material-extensions==1.0.3
 mkdocs-minify-plugin==0.5.0
+mkdocs-redirects==1.0.4
 mkdocs-simple-hooks==0.1.5
 nltk==3.7
 packaging==21.3