docs: Move the user-space ioctl() docs to userspace-api

This is strictly user-space material at this point, so put it with the
other user-space API documentation.

Signed-off-by: Jonathan Corbet <corbet@lwn.net>

Documentation/userspace-api/index.rst

@@ -21,6 +21,7 @@ place where this information is gathered.
    unshare
    spec_ctrl
    accelerators/ocxl
+   ioctl/index
 
 .. only::  subproject and html