manual: high-level restructuring

manual: high-level restructuring

The structure of the buildroot manual is not always clear. There is a large
number of chapters, and some chapters seem to overlap. The distinction
between general usage and developer information is not always clear.

This patch restructures the manual into four large parts:
- getting started
- user guide
- developer guide
- appendix

Except for the names of these parts, the section names are not yet changed.
Content-wise there are no changes yet either. This will be handled in
subsequent patches.

In order to achieve the introduction of a new level 'parts' above
'chapters', the section indicators (=, ==, ===, ...) of several sections
have to be moved one level down. Additionally, the leveloffset indication to
asciidoc has to be removed. Finally, to maintain more or less the same level
of detail in the table of contents, the toc.section.depth attribute is
reduced as well. Note that for some sections, less detail is visible now.

Signed-off-by: Thomas De Schampheleire <thomas.de.schampheleire@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>