docs: Fix subsystem APIs page so ungrouped entries have their own header

The kernel subsystem APIs front page currently has four top-level
groupings with headers, and then everything else that isn't grouped.
But in the table-of-contents, ungrouped subsystems are indented as
if they were part of the preceding grouping (currently "Storage
interfaces"), which is confusing.

Fix this by adding an "Other subsystems" header for the ungrouped
subsystems.

Fixes: 3c591cc9 ("docs: consolidate human interface subsystems")
Signed-off-by: Michael Kelley <mhklinux@outlook.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240125045941.123297-1-mhklinux@outlook.com