Add a link to Flink sql cookbook in the Aiven for Flink getting started page

[ftisiot]

What should we add to our content collections?

(more information is useful here! Were you trying to do something particular?)

It would be good to add a link to the Flink SQL cookbook in the Aiven for Apache Flink getting started page, or any page which talks about SQL

Is this something you would work on yourself?

[] I plan to open a pull request for this