manual: rework the whole documentation stub

The new skeleton of the manual as it has been thought:
1.  About Buildroot:
     Presentation of Buildroot
2.  Starting up:
     Everything to quickly and easily start working with Buildroot
3.  Working with Buildroot
     Basics to make your work fitting your needs
4.  Troubleshooting
5.  Going further in Buildroot's innards
     Explaination of how buildroot is organised, how it works, etc
6.  Developer Guidelines
7.  Getting involved
8.  Contibuting to Buildroot
9.  Legal notice
10. Appendix

It is easy to distinguish two parts in this plan:
- Sections 1 to 4 mainly address people starting with Buildroot
- Sections 5 to 10 are more focused on how to develop Buildroot itself

Most of the existing sections have just been moved in the hierarchy,
few were split and dispatch in, what i think was the relevant section,
and numerous others have been created.

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>