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_gtgt.md

@@ -0,0 +1,65 @@
+# <small>nlohmann::</small>operator>>(basic_json)
+
+```cpp
+std::istream& operator>>(std::istream& i, basic_json& j);
+```
+
+Deserializes an input stream to a JSON value.
+
+## Parameters
+
+`i` (in, out)
+:   input stream to read a serialized JSON value from
+
+`j` (in, out)
+:   JSON value to write the deserialized input to
+
+## Return value
+
+the stream `i`
+
+## Exceptions
+
+- Throws [`parse_error.101`](../home/exceptions.md#jsonexceptionparse_error101) in case of an unexpected token.
+- Throws [`parse_error.102`](../home/exceptions.md#jsonexceptionparse_error102) if to_unicode fails or surrogate 
+  error.
+- Throws [`parse_error.103`](../home/exceptions.md#jsonexceptionparse_error103) if to_unicode fails.
+
+## Complexity
+
+Linear in the length of the input. The parser is a predictive LL(1) parser.
+
+## Notes
+
+A UTF-8 byte order mark is silently ignored.
+
+!!! warning "Deprecation"
+
+    This function replaces function `#!cpp std::istream& operator<<(basic_json& j, std::istream& i)` which has
+    been deprecated in version 3.0.0. It will be removed in version 4.0.0. Please replace calls like `#!cpp j << i;`
+    with `#!cpp i >> j;`.
+
+## Examples
+
+??? example
+
+    The example below shows how a JSON value is constructed by reading a serialization from a stream.
+        
+    ```cpp
+    --8<-- "examples/operator_deserialize.cpp"
+    ```
+    
+    Output:
+    
+    ```json
+    --8<-- "examples/operator_deserialize.output"
+    ```
+
+## See also
+
+- [accept](basic_json/accept.md) - check if the input is valid JSON
+- [parse](basic_json/parse.md) - deserialize from a compatible input
+
+## Version history
+
+- Added in version 1.0.0. Deprecated in version 3.0.0.