docs(examples): add cache interceptor example with fetch

[nthbotast]

What

• add a dedicated Cache interceptor with fetch example in docs/examples/README.md
• show a minimal local server returning Cache-Control: public, max-age=60
• demonstrate that two consecutive fetch() calls return the same cached payload

Why

Issue #4283 asks for a clearer fetch-based example for the cache interceptor.

Validation

• git diff --check