media: docs: get rid of Documentation/media/

Now that everything got moved, we can get rid of the
old media directory.

Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>

Documentation/driver-api/index.rst

@@ -63,6 +63,7 @@ available subsections can be seen below.
    pinctl
    gpio/index
    md/index
+   media/index
    misc_devices
    nfc/index
    dmaengine/index