Mention `coverage` command as part of Overall workflow -> 3. Explore data

[yolile]

E.g. to say that the indicators require certain fields to exist and you can use this command to check what red flags are possible to calculate.

[jpmckinney]

Or #26 instead

[yolile]

Or both. I think some users could use cardinal for the coverage command only (as we do in some of our notebooks actually!), so it would be nice to mention it somewhere, as I think right now the coverage command lives in the menu only.