Add docs about mpirun launch vs. direct launch.

[qkoziol]

Add docs about mpirun launch vs. direct launch.

[qkoziol]

Split out from #10480

[qkoziol]

I reworked the "quickstart" info for mpirun, etc. as part of the earlier work to retire the FAQ: https://docs.open-mpi.org/en/v5.0.x/launching-apps/quickstart.html

I think it addresses this issue adequately, but more input would be welcome.

@jsquyres @gpaulsen @awlauria @janjust - I'm inclined to close this as completed. Any objections?

[janjust]

👍 looks good to me

[lrbison]

👍 looks good

[awlauria]

Looks great - thanks.