manual: provide make targets to build the documentation

Special thanks for Yann E. Morin for giving input and suggestions to
implement this.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Acked-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
[Peter: skip double mention of manual target in make help]
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>