Add ground as reference for NPX 2.0 probes

[bparks13]

See PR for details

[bparks13]

This issue is actually already completed due to how the docs are set up, so unless we want to add more details anywhere this will be closed.

For reference, in the GUI page I simply link out to the references page, which is automatically updated with the new XML comments and enum values, so no manual changes are needed unless we want to highlight Ground as an option.

[cjsha]

per tech talk, we are closing this as the other options for reference are not manually documented either on this page. Instead there is a link to the enum page that describes the reference options which is automatically generated from XML comments. So we agreed we can just close this without additional actions.