Add SPMC test documentations

Add build and run instructions for the SPMC test. Document the structure
of the SPMC test environment and the implemented test cases.

Signed-off-by: Imre Kis <imre.kis@arm.com>
Change-Id: I5139f36dc78a2c37c3000f15f74722f945ab738f
diff --git a/docs/developer/index.rst b/docs/developer/index.rst
index 4ea55f5..3204633 100644
--- a/docs/developer/index.rst
+++ b/docs/developer/index.rst
@@ -16,9 +16,10 @@
     docs-build
     writing-documentation
     b-test
+    spmc-tests
 
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-*Copyright (c) 2020-2022, Arm Limited and Contributors. All rights reserved.*
+*Copyright (c) 2020-2024, Arm Limited and Contributors. All rights reserved.*
 
 SPDX-License-Identifier: BSD-3-Clause