README: link to manuals index; demote package link
README: forgot to update instructions to version 3
GH-13: man(1) on Mac OS X could not display URLs

The version of groff on Mac OS X is too old: it lacks the an-ext.tmac
macro package that defines styles for email addresses and general URLs.

    $ groff --version
    ...
    GNU grops (groff) version 1.19.2
    GNU troff (groff) version 1.19.2

This patch drops those URL macros in favor of simple angle brackets.
references were escaped in heading permalink IDs