build and deploy ePub e-book files

[das-g]

~See a run of these actions here.~

~Results available from https://das-g.github.io/django-girls-tutorial/book_<lang>.epub, e.g. for English https://das-g.github.io/django-girls-tutorial/book_en.epub~

~Before merging this, we'll probably want to have more meaningful file names, e.g. django-girls-tutorial_en.epub.~

Results available from https://das-g.github.io/django-girls-tutorial/django-girls-tutorial_<lang>.epub, e.g. for English https://das-g.github.io/django-girls-tutorial/django-girls-tutorial_en.epub

closes #1785

[das-g]

Is there anything that links to it?

Not yet. But https://github.com/DjangoGirls/tutorial/blob/master/LANGS.md?plain=1 has the paths to the HTML versions of the various translations hardcoded anyway, so I guess we could also include hardcoded links to the ePub files?

[ekohl]

I think that's a good idea.