docs: add guidance for enabling UI route with Flex

[lacatoire]

Summary

• The installation docs only described route registration inside a "not using Flex" note block
• Flex users had no indication that the UI route in config/routes/nelmio_api_doc.yaml needs to be uncommented to browse the documentation
• Restructure the section so all users see the route configuration, with a separate note for non-Flex kernel registration

Fixes #2592

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 95.59%. Comparing base (e651ed9) to head (670c573).
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##              5.x    #2695   +/-   ##
=======================================
  Coverage   95.59%   95.59%           
=======================================
  Files          94       94           
  Lines        3064     3064           
=======================================
  Hits         2929     2929           
  Misses        135      135           

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