docs: clarify Moirai 2 multivariate tutorial support

[kiwoongyoon]

Title: docs: clarify Moirai 2 multivariate tutorial support

Summary

• Default the pandas tutorial to Moirai so it runs through the multivariate example.
• Document that Moirai 2 does not currently support multivariate forecasting and direct users to Moirai or Moirai-MoE.
• Checked the issue snapshot for assignment and duplicate-work signals; it is unassigned and no related PR is recorded.

Test evidence

• python3 -c "import json; n=json.load(open('example/moirai_forecast_pandas.ipynb')); s=''.join(''.join(c.get('source', [])) for c in n['cells']); assert 'MODEL = \"moirai\"' in s; assert 'Moirai 2 does not currently support multivariate forecasting.' in s; print('notebook guidance verified')" — verified notebook JSON, the supported default, and the multivariate guidance.
• git diff --check — passed.

Risks or notes for maintainers

• This is documentation/tutorial guidance only; it does not add Moirai 2 multivariate support.
• Full notebook execution was not run because it requires remote data and pretrained model downloads.
• Salesforce CLA completion and any code-owner review remain required before merge.

Refs #238

[salesforce-cla]

Thanks for the contribution! Before we can merge this, we need @kiwoongyoon to sign the Salesforce Inc. Contributor License Agreement.