Can I use Apple Watch fast charger in an Apple Watch version 5?

No such thing as a Fast Charger. Chargers just offer the power they are rated for.

Watches can then fast charge with that charger if they support the feature and the charger offers enough power to engage fast charging.

You can use a high powered charger to charge the watch sure, but the Series 5 watch will not charge faster than with a regular charger.

Only Series 7 and 8 support fast charging with high powered chargers as explained in the support document you posted from.