Restore demo library operation

The demo library needs to generate some files to work before starting the services.

It also needs access to github.com.

Procedure:

• check that configuration at https://github.com/kiwix/operations/blob/main/zim/demo-library/demos.yaml is OK (all ZIMs available on storage machine) or fix this
• deploy the cronjob zim/demo-library/demo-generator.cronjob.yaml and start it
  • do not mind about errors on cache purge if the script end with "Done"
• deploy the 3 services zim/demo-library/kiwiserve.yaml, zim/demo-library/staticserver.yaml and zim/demo-library/webcache.yaml