Docs: Add SPM backends introduction

Change-Id: I5f5c1678823e6095368fb8c7d99a1d8b7eac9f56
Co-authored-by: Ken Liu <ken.liu@arm.com>
Signed-off-by: Kevin Peng <kevin.peng@arm.com>
diff --git a/docs/integration_guide/index.rst b/docs/integration_guide/index.rst
index b68558c..3599754 100644
--- a/docs/integration_guide/index.rst
+++ b/docs/integration_guide/index.rst
@@ -8,6 +8,7 @@
 .. toctree::
     :maxdepth: 1
 
+    SPM Backends <spm_backends.rst>
     NS client integration <non-secure_client_extension_integration_guide.rst>
     OS migration to Armv8-M <os_migration_guide_armv8m.rst>
     tfm_fpu_support