docs: archis: add a per-architecture features list

Add a feature list matrix for each architecture to their
respective Kernel books.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
Link: https://lore.kernel.org/r/9c39d4dd93e05c0008205527d2c3450912f029ed.1606748711.git.mchehab+huawei@kernel.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/arm64/index.rst b/Documentation/arm64/index.rst
index 937634c..97d65ba 100644
--- a/Documentation/arm64/index.rst
+++ b/Documentation/arm64/index.rst
@@ -24,6 +24,8 @@
     tagged-address-abi
     tagged-pointers
 
+    features
+
 .. only::  subproject and html
 
    Indices