Distribute built documentation

This patch changes the way documentation is built in order to distribute
the generated output in the 'dist' archive, and thus no longer
requiring the presence of the docbook/rst toolchains during build time.
This will lower the requirements for installation and also makes the
build time insignificant.

First, we remove the docbook2pdf rules and variables, since we no longer
build this kind of docs. Furthermore, the rst source files are not
(today) processed via replace_vars_sed, so the whole .in rules for doc/
go away.

Next, we change the ".sgml|.rst -> replace_vars_sed -> .in -> processor
-> final file" processing to ".sgml|.rst -> generator -> .in ->
replace_vars_sed -> final file"; this means we first process the file
using the formatter, with the @VARIABLE@ entries in it, and save the
output as .in; this output we distribute, and on the user side, the
replace_vars_sed will use the new configure flags to transform the
(almost final .in form) to the final form, without needing the
toolchain.

In configure.ac we also change from ERROR to WARN for the documentation
generators, and extra tests in Makefile.am check that the programs have
been found.

This was tested with distcheck and works as expected.

Reviewed-by: ultrotter