Install man pages with selected version

rgbenv doesn't currently handle the rgbds man pages, so if you installed rgbds via rgbenv you can't access the manual in the usual way (`man rgbgfx`).

The pages from the selected rgbds version should be made available to man-db -- this will enable user to `man rgbgfx`, etc. for rgbds installed via rgbenv.

There's not much required to get this working -- man-db is friendly and will automatically discover pages in an appropriately named directory relative to PATH entries. I have implemented a solution in a modified rgbenv which I've been using for quite some time -- I will make a PR for this.