Get FreeBSD: section levels, and article title

AsciiDoc for an article type document (the default): fix the level of
each section below the sole section that is level zero (the title).

https://docs.asciidoctor.org/asciidoc/latest/syntax-quick-reference/#section-titles

The FreeBSD Project home page describes https://www.freebsd.org/where/
as 'Get FreeBSD', not 'Download FreeBSD'. Gain consistency in this page.

(Consistency in the FreeBSD Handbook and elsewhere may be gained through
future reviews.)

Approved by: gjb
Fixes: 5c8cc20333c4 Download FreeBSD: headings, redundancy, clarity
Differential revision: https://reviews.freebsd.org/D37307