Document the replay plugin setup for `adk conformance test`

[ftnext]

Describe the bug

The conformance testing documentation shows how to start adk web with RecordingsPlugin to create a baseline, but does not explain that replay testing requires restarting the server with ReplayPlugin.
Following the documented steps alone can leave replay recordings unloaded.

Replay verification failed: AttributeError: 'NoneType' object has no attribute 'recordings'

To Reproduce

1. Follow the documentation to create a baseline using RecordingsPlugin and adk conformance record.
2. Continue to the “Run conformance tests” section and run adk conformance test without changing the web server plugin.
3. Observe that replay cannot load the recordings.

Expected behavior

The documentation should instruct users to restart adk web with --extra_plugins=google.adk.cli.plugins.replay_plugin.ReplayPlugin before running replay tests, and show the matching adk conformance test command.

Screenshots

N/A

Versions

• OS: macOS
• ADK version: 2.9.2
• Python version: 3.13.12

Additional context

A walkthrough with the missing step is available in Japanese.