Clarify Docker webhooks setup

[Chandu0501]

Closes #259

Summary

• remove the outdated Webhooks JAR download step from the Docker guide
• explain that Webhooks are bundled in WireMock 3.1.0+ and only need --extensions
• keep the mount guidance for third-party/custom extensions and point legacy users to the 2.x docs

Verification

• pnpm.cmd install
• pnpm.cmd build rendered dist/docs/standalone/docker/index.html with the updated guidance
• pnpm.cmd build still exits non-zero afterwards because of the repo's pre-existing Windows copy-static-homepage hook bug (C:\C:\...\dist\index.html)