thunderbolt.4: Initial manual for HW Relnotes

This manual contains nothing and is only suitable for the HW Relnotes,
but lets get it in so we have something and then can iterate on it.

MFC after: 3 minutes
Fixes: 2ed9833791f2 (thunderbolt: Import USB4 code)
Discussed with: obiwac
Differential Revision: https://reviews.freebsd.org/D52847