missing doctests

[d-v-b]

I don't think we have doctests working -- the doctest commands were removed from pyproject.toml in our switch to mkdocs, and the CI doctest job silently does nothing. We should probably fix this.

[d-v-b]

this was partially addressed by #3500, but there are still many instances of code samples in our docstrings that have not been migrated to code-fenced markdown blocks. These all need to be converted before our doctests are complete.