Improve Stable Release doc

[lbdyck]

See https://github.com/zopencommunity/curlport/releases/tag/STABLE_curlport_3213

1. the sample curl command to download and install does not have a copy text option - would be helpful
   but
2. if the site does not have curl already installed then they should do a wget instead which is installed in the z/OS base and will also work
   and
3. The zopen install curl option is nice but a note should be made that zopen needs to have already been installed and also provide a link to where to and how to
4. If just downloading the ...pax.Z file it would be nice to have the appropriate pax command delineated along with the need for sourcing the .env

[lbdyck]

One more thought - any chance of having a generic (aka permanent) url for the latest stable release

And this applies, after I've looked, at other zopen repositories

[lbdyck]

My BAD - wget is part of zopen so that isn't a valid option if one does not have zopen or curl available.

Can ftp be used or is the only other valid option to use a browser to download?