Docs: Update code structure doc

- fix the number of CPU to 8 to avoid Sphinx crash in OpenCI

Signed-off-by: Anton Komlev <anton.komlev@arm.com>
Change-Id: I4f05857a639ec2609f5b21cb739a6e91e5372fc0
diff --git a/docs/platform/index.rst b/docs/platform/index.rst
index 871395b..3bce6f1 100644
--- a/docs/platform/index.rst
+++ b/docs/platform/index.rst
@@ -1,3 +1,5 @@
+.. _supported_platforms:
+
 ##############
 TF-M Platforms
 ##############