infra: rename the download-sources documentation file

The 'Download Sources' documentation file contains instructions on
setting up the workspace on this host machine (in addition to the
instructions on downloading the source code). So rename this file
name and all the references to this file.

Signed-off-by: Thomas Abraham <thomas.abraham@arm.com>
Change-Id: I7a87d0aa40b8564ab21fc68987197d95106d5f2f