docs: generate files into build dir and stop distributing them

Historically we did not support VPATH builds and everything was
generated into source directory. The introduction of VPATH builds did
not changed the way how our documentation is handled.

This patch changes the rules to generate everything into build
directory and stops distributing generated files in order to have
properly separated VPATH builds.

Signed-off-by: Pavel Hrdina <phrdina@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>