docs(dispatcher): add cache interceptor fetch example

[nthbotast]

What

• add a dedicated cache interceptor example for fetch in docs/docs/api/Dispatcher.md
• show how to compose a cached Agent, set it globally, and illustrate first/second request behavior

Why

• closes the documentation gap tracked in Clear example of using Cache Interceptor with Fetch #4283 by providing a discoverable copy/paste example

Validation

• git diff --check
• npx --yes markdownlint-cli2 docs/docs/api/Dispatcher.md (fails due to many pre-existing file-wide lint issues unrelated to this change)

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.13%. Comparing base (bfeacd0) to head (8e0eb28).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4870      +/-   ##
==========================================
- Coverage   93.14%   93.13%   -0.01%     
==========================================
  Files         109      109              
  Lines       34254    34254              
==========================================
- Hits        31905    31904       -1     
- Misses       2349     2350       +1     

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