Build pdf documentation

From @mchehab

But ignore warnings for now.

What about the kerneldoc test?

It is already there, I forgot to mention it. Sorry about that.

both html and pdf?

Only html. Do we need pdf?

Yes, but errors shouldn't be relevant enough to reject merges, as they may be caused by something outside our control, except if it worked before and the merge broke it.

Also, pdf output is very noisy: we can't really do much about most of the warnings.

Thanks, Mauro