doc: Use POD as source format and pod2man to generate ncdu.1

POD is somewhat more simple and flexible. I now use ncdu.pod to generate
a nicely formatted manual page on the ncdu homepage, rather than
displaying a rendering of ncdu.1 formatted in a monospace font.

The tarball will still contain an ncdu.1, so there's no extra dependency
on pod2man. (Unless you clone from git, since ncdu.1 isn't in the repo)