add support for MOBI and EPUB formats on release

[bataliero]

Hi

Thank you for all the great work you do on the Python Packaging User Guide. It really helps the Python community a lot!

I have a suggestion: could you add support for MOBI and EPUB formats? These formats are very handy for people who like to read on e-readers or mobile devices. It would make it easier to access the documentation offline and in a more comfortable way. Maybe releasing them together with the HTML format would be a good idea.

Thanks!

[Ijashhh]

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