RN: Rearrange files and restructure the sections

This change does not remove any text. It only moves around targeting a
better overall order. The changes are:

The file
* machines-maintainers.rst
--> is included in bsp-scope.rst

The files
* soc-hierarchy.rst
* kernel-linux.rst
* bootloaders.rst
* userspace-pkg.rst
* fsl-userspace-pkg.rst
* pkggrp-img.rst
--> are included in new source-code.rst

In addition, it changes some section levels. It means, what was section
may become chapter, what was section, may become sub-section,
sub-section may become sub-subsection.

The new (numbered only here) TOC looks like:
1. Introduction
    1.1. What is the FSL Community BSP
        1.1.1. Motivation
        1.1.2. What the FSL Community BSP is not
        1.1.3. What you can expect
        1.1.4. What the community expects from you
    1.2. Upstream
        1.2.1. Main branch names
        1.2.2. Upstreaming cycle
    1.3. The differences between FSL Community BSP and Freescale
Official Release
        1.3.1. Freescale Official Release
        1.3.2. FSL Community BSP
2. FSL Community BSP Scope
    2.1. Kernel Release Notes
    2.2. Different Product SoC Families
    2.3. Supported Board List
        2.3.1. Machine Maintainers
            2.3.1.1. Machines with maintainers
            2.3.1.2. Machines without a maintainer
3. Software Architecture
    3.1. SoC Hierarchy
    3.2. Linux Kernel
        3.2.1. Default Linux Providers
    3.3. Bootloaders
    3.4. User Space Packages
        3.4.1. Freescale User Space Packages
            3.4.1.1. Version by soc-hierarchy
            3.4.1.2. Hardware relation by soc-hierarchy
    3.5. PackageGroups and Images
        3.5.1. PACKAGEGROUPS
        3.5.2. Images
4. Test results
5. Acknowledgements
    5.1. Dizzy Source Code
6. Known Issues
    6.1. Open
    6.2. Closed

Signed-off-by: Daiane Angolini <daiane.angolini@freescale.com>