📚 DOCS: Add PDF format to the Read The Docs config

[jdillard]

That way users can preview what tabbed PDF output looks like in the official docs. This should help with questions about the current state PDF support, like in #57.

I haven't set up a RTD project yet, so I just went off the configuration docs, hopefully I got it right.

[codecov]

Codecov Report

Merging #116 (2672203) into master (45308dd) will not change coverage.
The diff coverage is n/a.

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pytests	97.16% <ø> (ø)

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[foster999]

Thanks @jdillard, this looks right to me 👍🏻

I considered adding ePUB and zipped HTML builds too, but we might as well wait to see if anyone would find that useful first.

[jdillard]

I thought that as well, but figured just taking a baby step might be best for now.

It's working! https://sphinx-tabs.readthedocs.io/_/downloads/en/latest/pdf/