doc: describe TPL/VPL/SPL boot

This is a stub describing how TPL, VPL, and SPL load the next boot stages
on a detail level for users.

For sure we will need a few patches on top to catch the whole complexity.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Paul Barker <paul.barker.ct@bp.renesas.com>