media: docs: move uAPI book to userspace-api/media

Since 2017, there is an space reserved for userspace API,
created by changeset 1d596dee3862 ("docs: Create a user-space API guide").

As the media subsystem was one of the first subsystems to use
Sphinx, until this patch, we were keeping things on a separate
place.

Let's just use the new location, as having all uAPI altogether
will likely make things easier for developers.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>
diff --git a/Documentation/media/index.rst b/Documentation/media/index.rst
index 0301c25..8118c2d 100644
--- a/Documentation/media/index.rst
+++ b/Documentation/media/index.rst
@@ -12,7 +12,7 @@
 .. toctree::
    :maxdepth: 2
 
-   media_uapi
+   ../userspace-api/media/index
    media_kapi
    dvb-drivers/index
    v4l-drivers/index