Add add WMS Link example to demo application

[astroidex]

Describe your feature request

• https://demo.mapbender.org/application/mapbender_user_yml

• https://github.com/mapbender/mapbender-starter/blob/develop/application/config/applications/mapbender_user.yaml

• in Sidepane About Mapbender: Please add at the bottom the link to add a WMS

• Use WMS and activate one Layer
  TopPlusOpen https://gdz.bkg.bund.de/index.php/default/wms-topplusopen-wms-topplus-open.html

• Also link to https://gdz.bkg.bund.de/index.php/default/wms-topplusopen-wms-topplus-open.html

This is the new feature in 4.1 #517