doc: reorganize developer.texi chapter hierarchy

Previously, the Developer Documentation <ffmpeg.org/developer.html>
contained a single chapter, "1. Developer Guide," with all content under
that single chapter. Thus the document structure was one level deeper
and more complicated than it needed to be.  It differed from similar
documents such as /faq.html, which have multiple chapters.

Eliminate the single chapter, and promote each section underneath to
chapter, and each subsection to section. Thus content and relative
structure remains the same, but the overall structure is simpler.
Anchors within the page remain the same.

Signed-off-by: Jim DeLaHunt <from.ffmpeg-dev@jdlh.com>
Signed-off-by: Timothy Gu <timothygu99@gmail.com>