docs: add custom interceptor example

[vibhor-aggr]

Fixes #3218.

This adds a custom interceptor example to the Interceptors API documentation. The new section shows the current Dispatcher.compose() pattern and demonstrates how to wrap and forward handler callbacks while measuring response timing.

It also links readers from the Dispatcher.compose() documentation to the new custom example and points from the example back to the full dispatcher.dispatch(options, handler) callback reference.

Validation:

• git diff --check
• npm run lint
• npm run test:typescript

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.44%. Comparing base (ecf714f) to head (cec5714).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #5548   +/-   ##
=======================================
  Coverage   93.44%   93.44%           
=======================================
  Files         110      110           
  Lines       37418    37418           
=======================================
  Hits        34967    34967           
  Misses       2451     2451           

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[mcollina]

lgtm