docs: Move arm-cpu-features.rst into the system manual

Now we have somewhere to put arm-specific rst documentation,
we can move arm-cpu-features.rst from the docs/ top level
directory into docs/system/arm/.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Niek Linnenbank <nieklinnenbank@gmail.com>
Message-id: 20200309215818.2021-5-peter.maydell@linaro.org

docs/system/arm/cpu-features.rst

@@ -1,11 +1,5 @@
+Arm CPU Features
 ================
-ARM CPU Features
-================
-
-Examples of probing and using ARM CPU features
-
-Introduction
-============
 
 CPU features are optional features that a CPU of supporting type may
 choose to implement or not.  In QEMU, optional CPU features have

docs/system/target-arm.rst

@@ -78,3 +78,9 @@ undocumented; you can get a complete list by running
    arm/stellaris
    arm/musicpal
    arm/sx1
+
+Arm CPU features
+================
+
+.. toctree::
+   arm/cpu-features