rofl-docs: Document persistent storage

[matevz]

In the Deployment section, mention that the persistent storage for your ROFL lives in serverdir/runtimes/images/{runtime_id}/rofl.{app_id} folder. e.g. /serverdir/runtimes/images/000000000000000000000000000000000000000000000000a6d1e3ebf60dff6c/rofl.rofl1qqn9xndja7e2pnxhttktmecvwzz0yqwxsquqyxdf.

This folder is generated when ROFL is executed for the first time and will remain intact even during ROFL upgrades and removal.

** Troubleshooting

Also add to troubleshooting messages like

{"component":"rofl.rofl1qrtetspnld9efpeasxmryl6nw9mgllr0euls3dwn","level":"info","module":"runtime/global","msg":"Error: writing blob: adding layer with blob \"sha256:9f202d637e1bbe0e48c7855d7872fa4ab33af88b61ef10d4cb6dd7caba0e2c8a\"/\"\"/\"sha256:b240b4f256e7bd304b5a1335b4bc73b47ce21aaf31bb1107452a89a101f50054\": readlink /storage/containers/graph/overlay/l: invalid argument","provisioner":"tdx-qemu","runtime_id":"000000000000000000000000000000000000000000000000a6d1e3ebf60dff6c","runtime_name":"","ts":"2025-02-25T13:44:47.05176383Z"}

Solution? Remove the storage.