Mark AddOpenApiForJsonApi with [Experimental]

[bkoelman]

Adds [Experimental] to the AddOpenApiForJsonApi extension method and adds a warning to the docs.
Replaces our own UnreachableCodeException with the built-in System.Diagnostics.UnreachableException.

QUALITY CHECKLIST

• Changes implemented in code
• Complies with our contributing guidelines
• Adapted tests
• Documentation updated

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 0.00%. Comparing base (d81abed) to head (55faea6).
Report is 3 commits behind head on openapi.

Additional details and impacted files

@@       Coverage Diff       @@
##   openapi   #1663   +/-   ##
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