memory_model(7): create and document pointer provenance

Add a skeleton manpage intended to describe the FreeBSD memory model.

To start out, add documentation of pointer provenance and a cross link
to atomic(9).

Provide some advice on preserving provenance in CHERI and reference
more detailed discussions elsewere.

Effort: CHERI upstreaming
Reviewed by: kib, adrian, markj, emaste
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D57812