Move the release engineering howto to the doc tree, and convert it to
DocBook SGML. This version includes additional content from my paper
that isn't in www/en/internal/releng.sgml. That file will be removed
once this shows up on the website properly.

PR: docs/32697
Suggested by: bmah, nik, others
TeX to DocBook Conversion by: Hiten Pandya <hiten@uk.FreeBSD.org>