docs: Document growing the root filesystem for base image authors #29

jmarrero asked for upstream bootc docs telling base image authors that disk images booted on larger disks (e.g. in the cloud) need the root grown, and how: cloud-init, Ignition-based systems (Flatcar, Fedora CoreOS), or a systemd service like Fedora/CentOS bootc's bootc-generic-growpart.service (gitlab.com/fedora/bootc/base-images, standard/; not in the minimal variants). Also: distributions may ship image variants without a solution, which then needs layering in.

Target: extend docs/src/bootc-sysroot.7.md § "Expanding the root filesystem", linked from bootc-compatible-images.7.md § "Suggested image content" and bootc-installation.7.md § "Provisioning and first boot". Context: bootc#2465 (filed on bootc against the base-images script).

First step (this item): fact-check how Flatcar (Ignition), Fedora CoreOS and cloud-init grow the root, including whether cloud-init's growpart/resizefs work on a bootc root, testing on a devspace VM where needed.