doc: Rootless lazy pulling with Podman, nerdctl and BuildKit

[ktock]

This commit adds docs about rootless lazy pulling of eStargz with Podman, nerdctl and BuildKit.

Experimental configuration of Podman + rootless Stargz Store is currently implemented using podman unshare for putting both of them into the same namespace.

$ podman unshare stargz-store --root $HOME/.local/share/stargz-store/data $HOME/.local/share/stargz-store/store &
$ podman pull ghcr.io/stargz-containers/python:3.9-esgz

Current limitation of this approach is --uidmap flag of podman run doesn't work.
So we mark this configuration as "experimental" and will work on further investigation and improvement for eliminating the limitation.

 $ podman run -d --uidmap 0:10000:5000 --rm ghcr.io/stargz-containers/alpine:3.10.2-esgz sleep 10000
 Error: creating container storage: creating an ID-mapped copy of layer "9e0573c62127729a89b3e728371ebe4bd4a77996ea57c8f06bf8760bac3cf16d": error during chown: storage-chown-by-maps: lchown bin/arch: no such device or address: exit status 1