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/doc/README.gpt b/doc/README.gpt
index 62013a3..facd7af 100644
--- a/doc/README.gpt
+++ b/doc/README.gpt
@@ -229,7 +229,7 @@
 
 The GPT functionality may be tested with the 'sandbox' board by
 creating a disk image as described under 'Block Device Emulation' in
-board/sandbox/README.sandbox:
+doc/arch/index.rst:
 
 =>host bind 0 ./disk.raw
 => gpt read host 0