docs(amu): add AMU documentation

This change adds some documentation on the AMU and its purpose. This is
expanded on in later patches.

Change-Id: If2834676790938d8da5ea2ceba37b674f6cc0f01
Signed-off-by: Chris Kay <chris.kay@arm.com>
diff --git a/docs/components/index.rst b/docs/components/index.rst
index f349d8d..e467516 100644
--- a/docs/components/index.rst
+++ b/docs/components/index.rst
@@ -7,6 +7,7 @@
    :numbered:
 
    spd/index
+   activity-monitors
    arm-sip-service
    debugfs-design
    exception-handling