we should have documentation about valid measure expressions

[hamilton]

We have zero documentation on this, nor do we have links from the metrics config. This would be quite useful for users, who will surely be confused.

[ericpgreen2]

A few ideas for it:

• We'll probably want a docs page for "Create your first dashboard", which goes through a basic end-to-end example. Given that page, a separate reference page specifically for "Creating metrics" - showing the world of possibilities - would be helpful
• One section would focus on Measures and provide at least 3-5 examples of evolving complexity
• Focus on one real world use case (e.g. metrics from an eCommerce business)

[hamilton]

closing this given the following page: https://docs.rilldata.com/defining-metrics