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/json_pointer/index.md

@@ -28,7 +28,7 @@ are the base for JSON patches.
 
 - [(constructor)](json_pointer.md)
 - [**to_string**](to_string.md) - return a string representation of the JSON pointer
-- [**operator string_t**](operator_string.md) - return a string representation of the JSON pointer
+- [**operator string_t**](operator_string_t.md) - return a string representation of the JSON pointer
 - [**operator/=**](operator_slasheq.md) - append to the end of the JSON pointer
 - [**operator/**](operator_slash.md) - create JSON Pointer by appending
 - [**parent_pointer**](parent_pointer.md) - returns the parent of this JSON pointer

docs/mkdocs/docs/api/json_pointer/operator_string_t.md

@@ -19,6 +19,13 @@ operator string_t() const
 }
 ```
 
+## Notes
+
+!!! warning "Deprecation"
+
+    This function is deprecated in favor of [`to_string`](to_string.md) and will be removed in a future major version
+    release.
+
 ## Examples
 
 ??? example
@@ -26,16 +33,16 @@ operator string_t() const
     The example shows how JSON Pointers can be implicitly converted to strings.
      
     ```cpp
-    --8<-- "examples/json_pointer__operator_string.cpp"
+    --8<-- "examples/json_pointer__operator_string_t.cpp"
     ```
     
     Output:
     
     ```json
-    --8<-- "examples/json_pointer__operator_string.output"
+    --8<-- "examples/json_pointer__operator_string_t.output"
     ```
 
 ## Version history
 
 - Since version 2.0.0.
-- Changed type to `string_t` in version 3.11.0.
+- Changed type to `string_t` and deprecated in version 3.11.0.