Add readme for new documentation framework (#919)

docs/documentation/cn/sql-reference/sql-functions/string-functions/ascii.md

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+# ascii
+
+## Description
+
+返回字符串第一个字符串对应的ascii 码
+
+## Syntax
+
+`INT ascii(VARCHAR)`
+
+## Examples
+
+```
+mysql> select ascii('1');
++------------+
+| ascii('1') |
++------------+
+|         49 |
++------------+
+
+mysql> select ascii('234');
++--------------+
+| ascii('234') |
++--------------+
+|           50 |
++--------------+
+```

docs/documentation/cn/sql-reference/sql-functions/string-functions/concat.md

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+# concat
+
+## Description
+
+将多个字符串连接起来, 如果参数中任意一个值是NULL，那么返回的结果就是NULL
+
+## Syntax
+
+`VARCHAR concat(VARCHAR,...)`
+
+## Examples
+
+```
+mysql> select concat("a", "b");
++------------------+
+| concat('a', 'b') |
++------------------+
+| ab               |
++------------------+
+
+mysql> select concat("a", "b", "c");
++-----------------------+
+| concat('a', 'b', 'c') |
++-----------------------+
+| abc                   |
++-----------------------+
+
+mysql> select concat("a", null, "c");
++------------------------+
+| concat('a', NULL, 'c') |
++------------------------+
+| NULL                   |
++------------------------+
+```

docs/documentation/cn/sql-reference/sql-functions/string-functions/concat_ws.md

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+# concat_ws
+
+## Description
+
+使用第一个参数作为连接符，将第二个参数以及后续所有参数拼接成一个字符串.
+如果分隔符是NULL，返回NULL。
+`concat_ws`函数不会跳过空字符串，会跳过NULL值
+
+## Syntax
+
+`VARCHAR concat_ws(VARCHAR, VARCHAR,...)`
+
+## Examples
+
+```
+mysql> select concat_ws("or", "d", "is");
++----------------------------+
+| concat_ws('or', 'd', 'is') |
++----------------------------+
+| doris                      |
++----------------------------+
+
+mysql> select concat_ws(NULL, "d", "is");
++----------------------------+
+| concat_ws(NULL, 'd', 'is') |
++----------------------------+
+| NULL                       |
++----------------------------+
+
+mysql> select concat_ws("or", "d", NULL,"is");
++---------------------------------+
+| concat_ws("or", "d", NULL,"is") |
++---------------------------------+
+| doris                           |
++---------------------------------+
+```