CI checkdoc step is just a no-op

[bcc32]

test.yml specifies a checkdoc step (whose errors are currently suppressed), but
at the moment it doesn't actually do anything (and fails silently) because
https://download.tuxfamily.org/user42/checkdoc-batch.el is a broken link.

[purcell]

We might consider adopting something like makem: https://github.com/alphapapa/makem.sh