📝 add more API documentation

doc/mkdocs/docs/api/json_pointer.md

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+# json_pointer
+
+```cpp
+template<typename BasicJsonType>
+class json_pointer;
+```
+
+## Template parameters
+
+`BasicJsonType`
+:   a specialization of [`basic_json`](basic_json/index.md)
+
+## Member functions
+
+- (constructor)
+- **to_string** - return a string representation of the JSON pointer
+- **operator std::string**- return a string representation of the JSON pointer
+- **operator/=** - append to the end of the JSON pointer
+- **operator/** - create JSON Pointer by appending
+- **parent_pointer** - returns the parent of this JSON pointer
+- **pop_back** - remove last reference token
+- **back** - return last reference token
+- **push_back** - append an unescaped token at the end of the pointer
+- **empty** - return whether pointer points to the root document