docs: add ContentTypeStorageInterface usage example

[KristianH]

Add documentation showing how to access the current ContentType on any route using ContentTypeStorageInterface. Includes a practical Twig extension example for rendering a global header with context-aware navigation and language switching.

[silasjoisten]

@edodusi maybe you find time to write blogpost on how to do a language switcher without an additional api request using the symfony bundle.

And maybe this is something for a cookbook?

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.57%. Comparing base (f376fdd) to head (c4ab100).

Additional details and impacted files

@@            Coverage Diff            @@
##                1.x     #104   +/-   ##
=========================================
  Coverage     94.57%   94.57%           
  Complexity      285      285           
=========================================
  Files            43       43           
  Lines          1069     1069           
=========================================
  Hits           1011     1011           
  Misses           58       58           

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