doc changes

doc changes
modified:   contrib/packages/guix/gnunet-doc.scm

adapt to the changes in this mixed commit

modified:   doc/.gitignore

add new files

modified:   doc/Makefile.am

make use of canonical, included, rules
generate html output with docstyle.css from taler exchange.

modified:   doc/chapters/installation.texi
new file:   doc/chapters/vocabulary.texi
modified:   doc/gnunet-c-tutorial.texi
modified:   doc/gnunet.texi
new file:   doc/docstyle.css

not really in use yet, but collected for later:
doc/gendocs.sh , doc/hacks.el