docs: Describe how to boot local builds

[cgwalters]

We want to make this use case feel awesome; today it doesn't for multiple reasons.

We probably want a bootc status --booted-image which would print the digested pull spec to start.

Then one could at least podman pull $(bootc status --booted-image).

But we clearly also want to streamline exporting from the local ostree storage back into a container, which has some downsides/bugs right now.

[jeckersb]

LGTM. I like bootc status --booted-image for ergonomics. Hopefully I can get #215 sorted out soon once all of the excitement of the week is over, then that fixes a lot of this.

[ericcurtin]

To workaround, no "bootc status --booted-image", I did this:

https://github.com/ericcurtin/dnf-bootc/blob/main/bootc.py

    def get_os_from_bootc_status(self):
        data = subprocess.run(["bootc", "status"], capture_output=True, text=True, check=True)
        data = yaml.safe_load(data.stdout)
        image = data['spec']['image']['image']
        return f"FROM {image}\n"

I notice there's a --booted option that didn't seem to make any difference to the output.