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

(cherry picked from commit 2f7a796b590e67c5d123f2b00b3aaf7ba7a32a13)