Update new-multiplayer-adapter template for jsPsych.multiplayer namespace

[htsukamoto5]

Summary

• Updates new-multiplayer-adapter's scaffolding template to call jsPsych.multiplayer.connect() instead of the now-removed jsPsych.pluginAPI.connect(), matching the top-level jsPsych.multiplayer namespace introduced in jspsych/jsPsych#3694.
• Changes are confined to the generated adapter's README, docs template, example HTML, and src/index.ts doc comment — no logic changes, since MultiplayerAdapter's interface (connect, push, getAll, get, subscribe, disconnect, participantId) is unaffected by the rename.

Test plan

• Ran new-multiplayer-adapter non-interactively to scaffold a test package and confirmed generated files reference jsPsych.multiplayer.connect()
• Repo-wide grep confirms no remaining references to pluginAPI.connect or communicate()

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