Download FreeBSD: headings, redundancy, clarity

https://www.freebsd.org/where/

Level 1 heading 'Download FreeBSD' is not perceptibly different from
the identically-worded visible title. This heading appears to be
redundant.

The very short 'Help to Test' section is no more than direction to
visit the download page. Self-referencing.

Generally, the page has become a slightly disorderly mix. Begin
improving this, partly through adjustments to heading levels.

Two new headings, level 1:

Production Quality

Development and Testing

… and so on.

Reviewed by: pauamma
Approved by: gjb (mentor)
Differential revision: https://reviews.freebsd.org/D37235