[#61] Publish content as epub and/or pdf

[smcvb]

This PR introduces some preliminary investigation / work around being able to generate an eBook, albeit .pdf, .epub or .mobi, from the Reference Guide. I have not set it so it's published on any of our websites (as I do not have the credentials for that), but did get it to build locally. To get to that point, I had to do a couple of things:

• Add more plugins, as those are needed by the eBook conversion tool, being hints and simpletabs.
• Install Calibre locally, as that's what the eBook conversion tool uses under the covers.
• Install the ebook-convert plugin through npm, like so: npm install ebook-convert.
• The plugin necessity was described on the GitBook eBook page, which also described how to generate a .pdf/.epub/.mobi file.
• Running the gitbook pdf ./ ./ref-guide.pdf command showed me that the tabs blocks didn't work for the conversion tool. Hence, I reverted back to the codetabs instead.

Last point of interest is that the Code Tabs quite logically did not work in the eBooks. Hence we might have to rethink either the the code tabs or the clarity of a published eBook.

[smcvb]

Discussed the necessity of this ticket, which we've deemed quite low. Will be closing this PR and issue for now as the requirement to provide an eBook is not overly evident at the moment.