The Navigo Decouverte covers the ile de France and so Bayeux which is in Normandy is not covered.
It will cover as many zones as you buy it for. If Versailles is the only spot outside Paris, it won't be economical to buy an ND for 5 zones (unless you are also using it in from the airport in which case it does)
The confusion about using it outside Paris only if it is monthly is related to the fact that if you have a monthly pass you can go anywhere in all zones on weekends if you have a Paris only pass i.e. zones 1-2. If you have a weekly pass paid for 5 zones, of course you can use it for all 5 zones. If you have a monthly pass you can use a zone 1-2 pass for all 5 zones for no additional charge on weekends.
Also if you have a zone 1-2 pass you can within 3 hours of a trip top it up for that trip only -- a parcours -- which gives you the trip at a discount. So for versailles you would buy the trip to and charge it on your Navigo just before travel and again just before travel home. This is probably more trouble than its worth for this inexpensive trip, but it starts to be worth it for trips to CDG or Disneyland which run about 16-18 round trip.