Deprecated behavior being used in manual

[molpopgen]

Here, for example.

Are we building the manual with warnings? (Yes, using -W, which means it should error out)
Are we building the manual with deprecation warnings? Seems to be b/c it shows up on that page.
So, why isn't this being treated as an error in CI when building the docs?

[molpopgen]

Aha, we need to have the config file read:

execute:
  execute_notebooks: force
  stderr_output: error

The last line will treat the presence of stderr in a cell as an error.