Add PDF build

[willingc]

We currently have an HTML zip that is downloadable from ReadTheDocs. It would be helpful to also build and offer a PDF for download. Possibly epub too.

[louisom]

For epub, it can generate by make epub; for pdf, there is a rst2pdf package can use.

On ReadTheDocs, advance setting can both enable pdf and epub to build and download.

[berkerpeksag]

What's the use case? Most of the devguide requires a computer and a python/cpython checkout to follow its content. I don't think it's suitable for a morning read on an ebook reader :) I'd prefer avoid using more RtD resources.

[willingc]

Some of us periodically download PDF and ebooks to use as a reference, especially when off-line or in areas with poor internet access. It's a simple addition that requires very little effort or resource. @berkerpeksag It could also help those with insomnia ;-)

[brettcannon]

I initially turned it off as I wasn't sure if anyone was going to want them, but now that we have a request I have ticked the boxes for PDF and epub.

[willingc]

Thanks @brettcannon 👍