Document transformer registration for dynamic ports

[Chandu0501]

Fixes #375.

Summary

• add an explicit transformer registration section to the Transforming Responses page
• show how to register a transformer on a WireMockServer created with dynamicPort()
• clarify when to use the instance DSL directly vs calling WireMock.configureFor(...) for the static DSL

Verification

• pnpm approve-builds --all
• pnpm build
• Astro generated /docs/extensibility/transforming-responses/index.html successfully
• the build still fails afterwards in the repo's existing Windows copy-static-homepage hook because it copies to C:\C:\...\dist\index.html