add back doctest

[lebensterben]

Recently we replaced cargo test by cargo nextest and it's vastly faster.
Unfortunately, it doesn't support doctest yet.

We should add a separate step in CI to run cargo test --doc.

[mre]

Oh, I did not know that. Here's the tracking issue: nextest-rs/nextest#16.
In the meantime let's add back a separate step for doctests, yeah.

[mre]

We're testing the docs again, thanks to a separate step. We can move to the proper solution once it's available upstream.