Refresh AWS platform documentation (chapel-lang#24550)

This PR is a near-complete rewrite of `doc/rst/platforms/aws.rst` to
reflect current best-practices for running Chapel on AWS, which is to
use ParallelCluster (`pcluster`). This document now describes how to
setup a `pcluster` for best results when using Chapel, as well as notes
on building Chapel in a good configuration.

This also contains some notes on running on instances with GPUs,
although my testing of this has been less thorough.

Built and checked docs locally

[Reviewed by @arezaii]