Documentation: gpu: nova-core: Document fwsec operation and layout

Add explanation of fwsec with diagrams. This helps clarify how the
nova-core falcon boot works.

Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>
Signed-off-by: Alexandre Courbot <acourbot@nvidia.com>
Link: https://lore.kernel.org/r/20250708-nova-docs-v4-7-9d188772c4c7@nvidia.com


Signed-off-by: Danilo Krummrich <dakr@kernel.org>