Simplify the directory structure for demo pages and integration tests

[marlo-longley]

Not strictly needed but I find this a lot simpler than what we have currently. Great if others agree, but I won't hold up the release for it!

__tests__/integration/mirador becomes __tests__/integration
demo .html pages are moved out of the __tests__/integration/mirador directory because they are no longer needed during testing after recent work to decouple.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.1%. Comparing base (8c7121d) to head (0c3bdec).

Additional details and impacted files

@@           Coverage Diff           @@
##            main   #4163     +/-   ##
=======================================
- Coverage   95.2%   95.1%   -0.1%     
=======================================
  Files        325     325             
  Lines      16154   16154             
  Branches    2546    2538      -8     
=======================================
- Hits       15376   15353     -23     
- Misses       773     796     +23     
  Partials       5       5             

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[lutzhelm]

There is a single line in the README about running single test files that should be changed.

[marlo-longley]

@lutzhelm thank you for the review. I changed the README line.