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/xtensa/index.rst b/Documentation/xtensa/index.rst
index 52fa04e..6995244 100644
--- a/Documentation/xtensa/index.rst
+++ b/Documentation/xtensa/index.rst
@@ -10,3 +10,5 @@
    atomctl
    booting
    mmu
+
+   features