doc: Isolate security-related build options

Reference security specific build options from the user guide.

Change-Id: I0e1efbf47d914cf3c473104175c702ff1a80eb67
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>
diff --git a/docs/process/index.rst b/docs/process/index.rst
index aa5d6bb..a870c8f 100644
--- a/docs/process/index.rst
+++ b/docs/process/index.rst
@@ -12,3 +12,4 @@
    coding-guidelines
    contributing
    faq
+   security-hardening