DOC add multi-output narx example

[MatthewSZhang]

Checklist

• Used a personal fork to propose changes
• A reference to a related issue: DOC Multi-Output NARX example #55
• A description of the changes: Add example for multi-output narx

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d4306c4) to head (32b9809).
Report is 56 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main       #64    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files            3         6     +3     
  Lines          110       476   +366     
==========================================
+ Hits           110       476   +366     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.