Add a warning regarding mixing ports(7) and pkg(8) to install software

I think it's something that has been missing from the handbook for a long
time.

Unfortunately, this patch does not explain all the subtleties of the ports
framework (like, e.g., that ports create packages during the build process
which are then installed via pkg). We should probably hint the reader about
it somewhere, but that's something for another patch. What's important here
is to warn new users about problems they might encounter.

Also, add some examples of working with quarterly with Subversion.

PR: 251195
Submitted by: Fred <fjs.usa01 gmail.com>