docs: update logo, add non-transparent layer to selkie

### What does this PR do?

I've noticed in our README our selkie isn't coloured in / transparent.

Updates the logo to add a white colouring-in.

### Screenshot / video of UI

<!-- If this PR is changing UI, please include
screenshots or screencasts showing the difference -->

### What issues does this PR fix or reference?

<!-- Include any related issues from Podman Desktop
repository (or from another issue tracker). -->

N/A

### How to test this PR?

<!-- Please explain steps to reproduce -->

See README, now it'll be coloured in.

Signed-off-by: Charlie Drage <[email protected]>