doc: Fix inaccuracy due to automatic conversion

Due to the automatic conversion from html to rst some
invisible characters got their way in. Causing some weird
behavior on odt. This patches fixes that.
Makefile: Add a doc target in the makefile

This commit adds a doc target in the makefile so
the user can generate the doc in various format based
on the rst2* utilities. Default DOC_FORMAT being pdf.

make DOC_FORMAT=pdf doc
make DOC_FORMAT=html doc