quickstart: add flux keygen for users running Flux for the first time

[SteVwonder]

Without this, a user will get flux-broker: zsecurity_comms_init: The directory '$HOME/.flux' does not exist. Have you run "flux keygen"?

[vsoch]

I'm grateful for this issue because I was looking for flux-keygen (per the docs here https://flux-framework.readthedocs.io/projects/flux-core/en/latest/man1/flux-keygen.html) and I couldn't find it but then I found this issue and found... flux keygen ! 🙌

[garlick]

flux keygen used to be needed if you were going to use Flux at all, but now it's only needed if you are setting up a system instance, e.g. that bootstraps from systemd.

Quickstart seems to be about PMI-launched Flux (where certs are now generated on the fly and public keys exchanged with PMI) so this can be closed.