docs: dont render image plots as pdf to avoid huge files, adapt Makefile
accordingly, sphinx' latexpdf uses pdf files so replace file extensions in
built tex output files before pdflatex, slightly higher dpi for png
images so tutorial pdf still looks good
docs: add downloadable link to Tutorial pdf (has to be built with "make
latexpdf" independently)

build has to be done like this for this to work:
 - make html
 - make latexpdf
 - make html
docs: adding docs-buildbot script
docs: add links to introduction ipy notebooks on tutorial page
Merge pull request #793 from megies/fix_doc_building

Fix doc building