feat(deployments): switch ocis_full example to bind-mounted ./data/ volumes

[LukasHirt]

Summary

• Switch the ocis_full deployment example from docker-managed named volumes to ./data/-relative bind mounts so persisted data is directly accessible on the host
• Add migrate-volumes.sh, a one-time helper that copies data from the old named volumes into the new bind-mount paths for existing deployments upgrading in place
• Update the example README and deployment docs accordingly

Test plan

• docker compose config validates cleanly for the full stack plus enabled web extensions
• Fresh install: docker compose up creates and populates ./data/* directories as expected
• Upgrade path: run migrate-volumes.sh against a deployment with existing named volumes (including ocis-apps and companion-data) and confirm data lands in the correct ./data/* paths
• migrate-volumes.sh runs successfully under both bash 5.x and bash 3.2 (stock macOS /bin/bash)

[update-docs]

Thanks for opening this pull request! The maintainers of this repository would appreciate it if you would create a changelog item based on your changes.

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[LukasHirt]

FYI @mmattel I keep this now as a draft. It's quite a bigger change so I need to do some more testing first and also I do not want to merge this before the Saturdays workshop so that it does not break anything for that (and does not force updating the current slides/notes).