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>
diff --git a/doc/arch/x86/index.rst b/doc/arch/x86/index.rst
index 3dc19d6..69db0a5 100644
--- a/doc/arch/x86/index.rst
+++ b/doc/arch/x86/index.rst
@@ -9,3 +9,4 @@
    :maxdepth: 2
 
    x86
+   manual_boot