Add docs for SQLSTATE

[dveeden]

• https://en.wikipedia.org/wiki/SQLSTATE

mysql> SELECT abc;
ERROR 1054 (42S22): Unknown column 'abc' in 'field list'

Here 42S22 is in the 42000 class that means "syntax error or access rule violation".

See also https://github.com/pingcap/tidb/blob/master/pkg/parser/mysql/state.go

Suggestion:

• Have a list of SQLSTATE categories
• Have a list of SQLSTATE codes used in TiDB
• Link on https://docs.pingcap.com/tidb/stable/error-codes and elsewhere where appropriate