doc: Move platform list to the Platform Ports index page

doc: Move platform list to the Platform Ports index page

The list of upstream platforms on the index page is growing
quite long, especially with all the FVP variants being listed
individually.

This patch leverages the "Platform Ports" chapter in the docs
table of contents to condense this information. Almost all
platform ports now have documentation, so the table of
contents serves as the list of upstream platforms by itself.

For those upstream platforms that do not have corresponding
documentation, the top-level "Platform Ports" page mentions
them individually. It also mentions each Arm FVP, just as
the index page did before.

Note that there is an in-progress patch that creates new
platform port documentation for the Arm Juno and Arm FVP
platforms, so this list of "other platforms" will soon be
reduced further as those platforms become part of the
table of contents as well.

Change-Id: I6b1eab8cba71a599d85a6e22553a34b07f213268
Signed-off-by: Paul Beesley <paul.beesley@arm.com>