docs: add an in-depth explanations of the build steps

The build process is quite complex: gcc is built three times, there are
two C library steps, there are those companion libraries...

People often wonder what all these steps do, and why they are needed.

Recently, someone proposed a tutorial on the crossgcc mailing list:
http://sourceware.org/ml/crossgcc/2011-01/msg00059.html

This meant that there was a need for such a tutorial, and explanations
on how a toolchain is built. So i decide to extend my answers:
http://sourceware.org/ml/crossgcc/2011-01/msg00060.html
http://sourceware.org/ml/crossgcc/2011-01/msg00125.html

into proper documentation in crosstool-NG.

Thanks go to Francesco for suggesting this. He has a fine tutorial
for beginners there:
http://fturco.org/wiki/doku.php?id=debian:cross-compiler

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>

 This is a somewhat ordered TODO list:

 Recuring tasks:

 - update versions for every tools...

 - update and/or upgrade all samples

 Non-recurring tasks:

 - update newlib (for enhanced bare metal)

   - confirm existing implementation on targets other than AVR32

   - try to make it generic, will help for uClibc++

 - multilib

 - Cygwin target (needs newlib)

   - so we can build toolchains that generate code for Windows/Cygwin

 - Cygwin host

   - so we can build toolchains that run natively under Windows/Cygwin

 - mingw32 target

   - so we can build toolchains that generate code for Windows/Win32

 - mingw32 host

   - so we can build toolchains that run natively under Windows/Win32

     (without requiring Cygwin)

 - uClibc++

   - see newlib, above

 - uClinux

   - check whether we can use newlib under uClinux (?)