x86: doc: Split out manual booting into its own file

Move this out of the main file since for simple users it is easier to
rely on standard boot.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>

x86: doc: Move into its own directory

There is enough material that it makes sense to split this up into
several files. Create an x86/ directory for this purpose.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>