doc: Make it easier to add a new doxygen page

The repetitive parts of generating the server and client documentation are
merged, so it is easier to add another doxygen chapter: add a new line to
$publican_sources in publican/Makefile.am, and a list of C source files to
doxygen/Makefile.am.

Reviewed-by: Bryce Harrington <bryce@osg.samsung.com>