update docker examples for Varnish3, remove obsolete 'version' yaml

[polyester]

Description

• remove obsolete 'version: 3' statements from docker-compose.yml files
• syntax change in Varnish v3: "Headers" statement needs to become "Header"
• make clear that for ZEO setups, it is necessary to set ZEO_SHARED_BLOB_DIR: on, otherwise the backend container will create its own filestorage/blobstorage setup
• add an explicit ZEO backend with persistent storage to the 'traefik-volto-plone-backend' example as otherwise the data lives in a hard-to-find randomly named volume

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📚 Documentation preview 📚: https://plone6--1963.org.readthedocs.build/

[jensens]

Technically fine. Good to don't call it "very simple" any more. I would go further for "Basic" or alike, it's not "Simple", nothing is simple if Varnish comes into the system.

[stevepiercy]

Thanks for these corrections! I made a few English grammar fixes.

[stevepiercy]

Thank you!