Fix GitHub action for deploy-pages

[doyled-it]

See error here. The error occurs when running sphinx-build:

Extension error:
Could not import extension sphinx_copybutton (exception: No module named 'sphinx_copybutton')

And when I search for this issue, the most applicable that I find is here:

executablebooks/sphinx-copybutton#169

It doesn't make sense because sphinx-copybutton is installed both locally when I try to run this command and in the Action.

[doyled-it]

Could be done together with #23