Update demo server configuration

[clarasb]

Adds demo cube to server configuration of panels demo, to demonstrate the handling of multi-dimensional data cubes in xcube-viewer (xcube-dev/xcube-viewer#612)

Checklist:

• Add unit tests and/or doctests in docstrings
• Add docstrings and API docs for any new/modified user-facing classes and functions
• New/modified features documented in docs/source/*
• Changes documented in CHANGES.md
• GitHub CI passes
• AppVeyor CI passes
• Test coverage remains or increases (target 100%)

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.60%. Comparing base (a62a48b) to head (0e607ae).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1222   +/-   ##
=======================================
  Coverage   89.60%   89.60%           
=======================================
  Files         280      280           
  Lines       21693    21693           
=======================================
  Hits        19437    19437           
  Misses       2256     2256           

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