Add Brienz-Rothorn-Bahn

[MoritzWeber0]

https://discord.com/channels/1250522473188032512/1532680415196745838/1532680883801292811

[MoritzWeber0]

Not part of SP, BLS or SBB though (as far as I can see). Maybe we add it to the Switzerland country page? Or just to the SP page with the hint that coupons are not valid?