Added introductory documentation about capsule support for TI devices,
including links to more detailed information.

Also added a note in the build secction that points to the host package
dependency docs.

This patch is followup from a request in the series introducing capsule
update for TI boards.
https://lore.kernel.org/r/20240618145058.552eapp5iiz772ej@hardcore



Signed-off-by: Jonathan Humphreys <j-humphreys@ti.com>