devel/libzim: Better URL and description; Python depedency at build only

We did not bump PORTREVISION after the removal of Python dependency at
runtime as most installations are expected to pull devel/libzim as
a dependency of www/kiwix-tools, which itself requires Python at
runtime. Moreover, we expect most machines running OpenZIM/Kiwix
software to have Python installed for other reasons. Folks who really
want, and can, get rid of the runtime dependency on Python still can
reinstall manually.

While here, expand a bit the description.

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