docs-ci: Make the Conda setup optional

[victorlin]

From #20 (review):

Make the Conda setup optional. Not all our repos use Conda for the docs build, and it my experience it seems to result in slower docs builds than pip. (IIRC, this also means we'd want to make the run.shell bit conditional on setup-miniconda, as that's the thing that restores -euo pipefail to the shell invocation another way.)