fix(convertor): add overlaybd vsize option

[fourierrr]

What this PR does

Adds an --overlaybd-vsize option to nerdctl image convert --overlaybd.

The option defaults to 64, matching the standalone overlaybd convertor default, and is passed through types.OverlaybdOptions to overlaybdconvert.WithVsize(...).

This also updates the command reference and OverlayBD documentation.

Why this is needed

nerdctl image convert --overlaybd can not work fine now.

time="2026-06-08T08:34:38Z" level=fatal msg="failed to preprare snapshot \"overlaybd-conv-917563811-qune\": failed to attach and mount for snapshot 26: failed to mkfs for dev /dev/sdd: mke2fs 1.47.0 (5-Feb-2023)
mkfs.ext4: Device size reported to be zero.  Invalid partition specified, or partition table wasn't reread after running fdisk, due to a modified partition being busy and in use.  You may need to reboot to re-read your partition table.
: exit status 1: unknown"

Related context

containerd/accelerated-container-image#245
containerd/accelerated-container-image#256