Install man pages with selected version #19

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.