Add a man page and install it on Linux (fixes #357)

This commit adds a man page to $prefix/share/man/man6/opendungeons.6 ("6" being the category for games man pages).
I added the paths to the installation and doc folders in the man page, so it has to be configured via CMake (that's debatable, I can also drop the mention and make it a static file in dist/).

I also added instructions to install CREDITS and FAQ.txt to the typical doc dir on Linux: $prefix/doc/<name>. I did not install the README since it's aimed at developers (describes the source code and how to compile) and the source is not included in system installations).