Make it clear when one should import 'Servant' or 'Servant.API' in the tutorial

[alpmestan]

These past few days, a few folks asked why "import Servant" doesn't work when just depending on servant. We should make it clear quite early in the tutorial that Servant is the go-to module when writing a webapp and exports servant's combinators as well as all the server-side machinery, hence belonging to servant-server, whereas Servant.API "just" exports all the type-level combinators.