docs: restructure design section

Design section was renamed "Secure Parition Manager".
It is the section that details Hafnium's implementation
as the SPM.

Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I15c5591241ca41412e6520321e2bfacb7ec64590
diff --git a/docs/index.rst b/docs/index.rst
index d29a909..87b536b 100644
--- a/docs/index.rst
+++ b/docs/index.rst
@@ -8,7 +8,7 @@
    Home<self>
    about
    getting_started/index
-   design/index
+   secure-partition-manager/index
    threat_model_spm
    change-log
    appendix