docs/interop: Change the vnc-ledstate-Pseudo-encoding doc into .rst

The file seems to contain perfectly valid rst syntax already, so
rename it to .rst and wire it up in the index.

Message-Id: <20221213101806.46640-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>

docs/interop/index.rst

@@ -23,3 +23,4 @@ are useful for making QEMU interoperate with other software.
    vhost-user-gpu
    vhost-vdpa
    virtio-balloon-stats
+   vnc-ledstate-pseudo-encoding