makefile: fix install-doc

makefile: fix install-doc

During the documentation overhaul in
85a7c3742f06ab55fdf523a7a6a9cfe5cda09837,
generating the man page symlinks was automated
(introduction of doc/gen_doc.sh, now called doc/doc_gen.sh).

However, the install-doc target of the makefile fails to account for
the source folder not necessarily existing.
This change runs doc/doc_gen.sh before attempting to install
the man pages.

This has the questionable side effect of creating a folder as root and
creating a bunch of files as root (including the HTML files,
i.e. running mandoc as root) when doing sudo make install,
but the average user will "just" install and forget about it anyway.