fdo#80367 (part) help pages fo Calc XML source

Implement help button for Calc Data-XML Source

Change-Id: Ic1097b13c1d2266a8a1f21a4cbc9d416aacba892
Reviewed-on: https://gerrit.libreoffice.org/11055
Reviewed-by: David Tardon <dtardon@redhat.com>
Tested-by: David Tardon <dtardon@redhat.com>