Add a category for bootable container images

[castrojo]

Is your feature request related to a problem? Please describe.
Bootc is coming into the CNCF as sandbox: https://github.com/containers/bootc
The big feature here is that it is an OCI container image, but it includes a kernel and operating system, so they are designed to be a full OS vs. a application container.

Describe the solution you'd like
A category for bootc enabled images, bootc sounds great as a slug with "Bootable Containers" being the descriptive text.

Describe alternatives you've considered
We can just list them under containers, however it'd be useful to list the ones that are bootable since they are designed to be splatted onto instances and hardware.

Additional context
In the future we may want to think about a way to signify base images, I'd love to approach CentOS for example and have them publish base images directly.