docs/system/arm/aspeed: add the catalina-bmc machine

The Facebook Catalina BMC machine is supported by hw/arm but has never
been listed in the Aspeed documentation. Add it to the AST2600 machine
table.

While at it, shorten the document title. Listing every machine name in
the title duplicates the tables right below it, and made the title (and
its underline) grow past 350 columns. The machine list stays available
where it is readable, in the per-SoC tables.

Reviewed-by: Jamin Lin <jaminlin1207@gmail.com>
Reviewed-by: Cédric Le Goater <clg@redhat.com>
Signed-off-by: Emmanuel Blot <emmanuel.blot@free.fr>
Link: https://lore.kernel.org/qemu-devel/20261015-bmc-minerva-v3-1-7d50c8a108e5@free.fr
Signed-off-by: Cédric Le Goater <clg@redhat.com>