development(7): redirect users from hier(7)

The layout of the source tree is now only described in README.md. Retain
the cross-reference to hier(7) in SEE ALSO; it is still useful to
readers.

Reviewed by: imp, emaste
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D37136