docs/demos: Provide an online demo of Forklift

[ethanjli]

Currently the only demos available of Forklift require either having a local VM or having a dedicated Raspberry Pi. It would be nice if we could provide a "Play with Docker"-style demo of Forklift.

[ethanjli]

Since we need Forklift to talk to a Docker daemon and we need to be able to reboot in the demo, we should probably fork https://github.com/lxc/incus-demo-server to make a Forklift-specific demo based on the online Incus demo system. Here are some leads on setting up the necessary infrastructure:

• Digger (https://github.com/diggerhq/digger) for running Terraform on GitHub via a PR-driven workflow (instead of using Terraform Cloud), and https://docs.digger.dev/getting-started/github-actions-and-gcp if we'll host it on Google Cloud Platform
• https://github.com/sargassum-world/infra-foundations for Terraform config files to use with Digger
• https://linuxcontainers.org/incus/docs/main/installing/
• https://github.com/ublue-os/ucore