Rename $PYTHON_VERSION to $PACKAGE_VERSION; this is the version number for

Rename $PYTHON_VERSION to $PACKAGE_VERSION; this is the version number for
whatever is being documented, not necessarily Python.  (These tools are
also used for the How-To documents, etc.)

get_version_text():  New function.  Returns HTML fragment describing the
        software version and documentation date.

bot_navigation_panel():  Include the result of get_version_text() at the
        end, so the information is available on every page.