Update .readthedocs.yml

[jonesnoaht]

It would be nice to have PDF and EPUB outputs.

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[silky]

Thanks @jonesnoaht ! :)

@shlevy - Do you know if more needs to be done here to make these available from the docs sites?

[shlevy]

@silky @michaelpj has been managing the docs site.

[michaelpj]

Apparently it's already enabled in the RTD settings, although putting it here is clearer. I have no idea where you get them from, though.

[silky]

Merging then to see what happens.

Thanks @jonesnoaht :) If you are able to confirm that this has achieved what you want, that would be great!