Update Global/Syncthing.gitignore

[IbisLiven]

added missing ignore for .stfolder directories created by Syncthing

Reasons for making this change:

Syncthing creates .stfolder directories alongside .stversion directories by default (they call it a "folder marker").

Relationship to the project: (which project?) I use Syncthing sometimes, I use these templates and this is my first PR here. Expectations from this change: I'd like both to be ignored in the template, I don't expect to get anything beyond that.

Links to documentation supporting these rule changes:

Syncthing FAQ Syncthing Configuration docs