docs: clarify JUnit Jupiter imports

[Chandu0501]

Summary

• add an explicit import block for @WireMockTest and WireMockRuntimeInfo
• make the first JUnit Jupiter example copy-pasteable
• clarify the package users should import from when following this page

Fixes #304

Verification

• pnpm exec astro build generated /docs/junit-jupiter/index.html but still hit the repo's pre-existing Windows copy-static-homepage failure with an invalid C:\C:\... copy target
• mvn org.apache.maven.plugins:maven-dependency-plugin:3.8.1:get "-Dartifact=org.wiremock:wiremock:3.13.2"
• verified WireMockTest.class and WireMockRuntimeInfo.class exist in org.wiremock:wiremock:3.13.2