Add end-user manual in PDF

[nichtich]

This is already implemented (run make pdf in docs/*/), but

• layout needs to be adjusted
• does not run on travis because it takes too long (see Add end-user manual #337)
• split end-user manual from the rest of documentation (Split developer documentation and user manual #349)

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We could make the travis job only run on releases although this adds another fragile step to the release process. At least Pandoc 2.7 is required which may require different setup at travis.

German end-user manual is already finished, see https://coli-conc.gbv.de/cocoda/dev/user-manual-de.html.

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The PDF manual is only build if the docs/ folder was modified.

• This should be extended to tagged commit (releases) and to changes in the public/img/ folder.
• Installation of LaTeX could be cached as described here to save 30s-60s build time
• pdfbook is not available at travis (should be part of texlive package pdfjam)

[nichtich]

I've set up cached LaTeX installation on the travis branch. The PDF should be build when the docs/ folder was changed and on tagged commits. It looks like there is a new cache for each tag (?), so releases would need longer. Build still fails because pdfbook is missing (should be part of texlive package pdfjam)