doc: arch: sandbox: Replace all the instances of README.sandbox

commit 49116e6d236d ("doc: arch: Convert README.sandbox to reST")
Moves README.sandbox to doc/arch.
Replace all the existing instances to point to the right documentation
file.

Signed-off-by: Keerthy <j-keerthy@ti.com>
diff --git a/README b/README
index 7e610a8..d2f466e 100644
--- a/README
+++ b/README
@@ -212,7 +212,7 @@
 specific to be undertaken on a native platform. The sandbox is also used to
 run some of U-Boot's tests.
 
-See board/sandbox/README.sandbox for more details.
+See doc/arch/index.rst for more details.
 
 
 Board Initialisation Flow: