doc: preserve paraghraphs in nmcli man pages

Improve documentation by preserving paragraphs in the
nm-settings-nmcli man pages.

To do that structure of src/libnm-client-impl/nm-settings-docs-gir.xml
was changed to have "description" as subnode to property node instead
of attribute of property node. Another subnode "description-docbook"
was added - this node is then used when generating man pages.

tools/ and man/nm-settings-dbus.xsl
were also changed to accomodate for changes mentioned above.

Replace xsltproc tool with python script when generating

Deleted settings-docs.xsl since it was replaced by python script.

Change src/libnmc-setting/ accodring to newly
generated src/libnmc-setting/settings-docs.h
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