erlang: Add manpages to all release packages.

For review by @the-kenny, as he's the maintainer of Erlang in `<nixpkgs>`.

This puts manpages into `$out/lib/erlang/man` and also symlinks them into `$out/share/man` with an `erl` suffix (similar to how Debian is handling this) so those manpages are unlikely to collide with any other manpages, like for example `inet(3)`.