Add mention of `ncs` to documentation of Nkeys (in addition to `nk`)

[ronen]

Looking at the documentation of Nkeys here: https://docs.nats.io/running-a-nats-service/configuration/securing_nats/auth_intro/nkey_auth
it says:

To generate nkeys, you'll need the nk tool.

and similarly the README of the nkeys repo says:

nk - Command Line Utility

Located under the nk directory

I tried to install nk using homebrew, but I couldn't find it. Searching online I found this issue: Add nk Formula, which was closed with this comment:

You can just nsc for this purpose nsc generate nkey -h

Great! I already have nsc installed. But since nsc seems to be the (or at least a) recommended way to generate Nkeys, it would be nice to add it to the documentation it the above places.

Thanks!