Use compile-only checking

[alexandru]

We can use mdoc:compile-only for code snippets. This should speed up the building of the site.

I see no reason for generating output, because it's slow and I've never seen the need for it, other than for simulating repl sessions, but those are awkward when copy/pasting, as you're copy/pasting the output as well. If we need output, we can add it manually in comments. If we need types, we can add them explicitly.

See scalameta/mdoc#366