docs: split up the driver book

We don't need to keep it as a single large file anymore; split it up so
that it is easier to manage and the individual sections can be read
directly as plain files.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>
diff --git a/Documentation/index.rst b/Documentation/index.rst
index 0d6992b..9fe5e0c 100644
--- a/Documentation/index.rst
+++ b/Documentation/index.rst
@@ -13,7 +13,7 @@
 
    kernel-documentation
    dev-tools/tools
-   driver-api/drivers
+   driver-api/index
    media/index
    gpu/index