doc: drop latex support, add pdf support

latexpdf fails with error message "Too deply nested".
We want to avoid reducing the nesting level of doc strings
in frappy.lib.classdoc (less nice output) or a level of
nesting in method doc strings.

- latex removed from Jenkinsfile
- added support for rst2pdf

Change-Id: Ieb3355ef506e636e7e43a726c68327e3b1154469
Reviewed-on: https://forge.frm2.tum.de/review/c/secop/frappy/+/32406
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Enrico Faulhaber <enrico.faulhaber@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

requirements-dev.txt

@@ -3,6 +3,7 @@ sphinx_rtd_theme
 Sphinx>=1.2.1
 # for generating docu
 markdown>=2.6
+rst2pdf
 # test suite
 pytest
 pytest-randomly>=1.1