Documentation/00-index: update for new core-api folder

Signed-off-by: Silvio Fricke <silvio.fricke@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 85a6627..3bb82fb 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -53,6 +53,7 @@
    :maxdepth: 2
 
    driver-api/index
+   core-api/index
    media/index
    gpu/index
    80211/index