Remove references to alpha and SWIG. Change the list of configs to be a
list of configs and not ISAs even though they are frequently named after
the ISA, say that the options are in build_opts, add in missing top
level directories, update and generalize the description of full system,
re-wrap some text.

Change-Id: Id2fb9323dcb9d8bf8e86c78fb5e9aa6afed1c5b8
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50755
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>