Enhancement for Dynamic SQL section

[rcbirsan]

This is an enhancement proposal for Dynamic SQL - https://github.com/insum-labs/plsql-and-sql-coding-guidelines/tree/master/docs/4-language-usage/6-dynamic-sql.
I suggest adding explicitly the rule Use bind variables syntax for dynamic SQL.
While the rule is implicitly suggested by the two examples that we already have in this section, I think it would be useful to have it written clear, with good and bad examples, in our coding guidelines.