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/index.rst

@@ -57,7 +57,6 @@ the kernel interface as seen by application developers.
    :maxdepth: 2
 
    userspace-api/index
-   ioctl/index
 
 
 Introduction to kernel development

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