1. move `group_concat` from string-functions to aggregate-functions.
2. add `json_array`/`json_object`/`json_quote` to sidebar file.
3. move `json_array`/`json_object`/`json_quote`/`get_json_double`/`get_json_int`/`get_json_string` to json-functions.
4. change `group_concat` document to uppercase