Contributors to FreeBSD: more relevance and appeal

Don't begin the article with the one and only section that is (much)
more than twelve years out of date.

Move the Donors gallery to the end. Improve it a little –
not forgetting a link out to The FreeBSD Foundation.

Approved by: doc (pauamma, carlavilla)
Differential revision: https://reviews.freebsd.org/D38796