Document .vnc file format in vinagre(1) manpage

Fixes https://bugzilla.gnome.org/625879 (I didn't find a corresponding gitlab issue).

Reverse-engineered from source. Not sure it deserves a separate vnc(5) page, it's not like this format is inter-operable with other software? WDYT?

• I'm not certain about case of [Options] and Shared= because impl_new_connection_from_file() looks them up capitalized, same as all others, but vnc_fill_conn_from_file() looks them up lowercased!

  Can that be a bug? I see in practice vinagre --file=foo.vnc does treat group & key names as case-sensitive. No idea when the latter function comes into play.

The general GKeyFile format is documented more fully at https://developer-old.gnome.org/glib/2.68/glib-Key-value-file-parser.html#glib-Key-value-file-parser.description which in turns references https://specifications.freedesktop.org/desktop-entry-spec/latest/ar01s03.html (but that contains irrelevant requirements specific to .desktop files)

rendered in terminal	man -Tdvi data/vinagre.1 > vinagre.dvi && dvipdfmx vinagre.dvi -o vinagre.pdf