Docs: Add or link to worked examples in expressions content

Feedback about the [Write expression queries](https://grafana.com/docs/grafana/latest/panels-visualizations/query-transform-data/expression-queries/#about-expressions) page: I think the customer is after some worked examples? In their case, they want to use expressions to calculate  values from both metrics and logs, but I'm sure there are other examples we could provide, even if it's a link from there to a dashboard on [play.grafana.org](http://play.grafana.org/) that demonstrates the various options