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/operator_ltlt.md

@@ -0,0 +1,86 @@
+# <small>nlohmann::</small>operator<<(basic_json), <small>nlohmann::</small>operator<<(json_pointer)
+
+```cpp
+std::ostream& operator<<(std::ostream& o, const basic_json& j);      // (1)
+
+std::ostream& operator<<(std::ostream& o, const json_pointer& ptr);  // (2)
+```
+
+1. Serialize the given JSON value `j` to the output stream `o`. The JSON value will be serialized using the
+   [`dump`](basic_json/dump.md) member function.
+    - The indentation of the output can be controlled with the member variable `width` of the output stream `o`. For
+      instance, using the manipulator `std::setw(4)` on `o` sets the indentation level to `4` and the serialization
+      result is the same as calling `dump(4)`.
+    - The indentation character can be controlled with the member variable `fill` of the output stream `o`.
+      For instance, the manipulator `std::setfill('\\t')` sets indentation to use a tab character rather than the
+      default space character.
+2. Write a string representation of the given JSON pointer `ptr` to the output stream `o`. The string representation is
+   obtained using the [`to_string`](json_pointer/to_string.md) member function.
+
+## Parameters
+
+`o` (in, out)
+:   stream to write to
+
+`j` (in)
+:   JSON value to serialize
+
+`ptr` (in)
+:   JSON pointer to write
+
+## Return value
+
+the stream `o`
+
+## Exceptions
+
+1. Throws [`type_error.316`](../home/exceptions.md#jsonexceptiontype_error316) if a string stored inside the JSON
+   value is not UTF-8 encoded. Note that unlike the [`dump`](basic_json/dump.md) member functions, no `error_handler` can be set.
+2. None.
+
+## Complexity
+
+Linear.
+
+## Notes
+
+!!! warning "Deprecation"
+
+    Function  `#!cpp std::ostream& operator<<(std::ostream& o, const basic_json& j)` replaces function
+    `#!cpp std::ostream& operator>>(const basic_json& j, std::ostream& o)` which has been deprecated in version 3.0.0.
+    It will be removed in version 4.0.0. Please replace calls like `#!cpp j >> o;` with `#!cpp o << j;`.
+
+## Examples
+
+??? example "Example: (1) serialize JSON value to stream"
+
+    The example below shows the serialization with different parameters to `width` to adjust the indentation level.
+    
+    ```cpp
+    --8<-- "examples/operator_ltlt__basic_json.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator_ltlt__basic_json.output"
+    ```
+
+??? example "Example: (2) write JSON pointer to stream"
+
+    The example below shows how to write a JSON pointer to a stream.
+    
+    ```cpp
+    --8<-- "examples/operator_ltlt__json_pointer.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator_ltlt__json_pointer.output"
+    ```
+## Version history
+
+1. Added in version 1.0.0. Added support for indentation character and deprecated
+   `#!cpp std::ostream& operator>>(const basic_json& j, std::ostream& o)` in version 3.0.0.
+3. Added in version 3.11.0.