intro.5: Add local to files, minor maintenence

New users are sometimes confused about the difference between /etc/ and
/usr/local/etc. Explain this in the manual as we did in intro(1). Link
hier(7) which now lists base system configuration files in /etc/. Add a
section number to HISTORY for clarity.

PR: 248562
MFC after: 3 days
Approved by: mhorne (mentor)
Reviewed by: imp, mhorne, Lexi Winter <lexi@le-fay.org>
Pull Request: https://github.com/freebsd/freebsd-src/pull/1534

(cherry picked from commit 8d65152cbfc8861f6920846dea6f540c673ab7b6)