docs: add tutorial for callable parameters

[AayushSabharwal]

Checklist

• Appropriate tests were added
• Any code changes were done in a way that does not break public API
• All documentation related to code changes were updated
• The new code follows the
  contributor guidelines, in particular the SciML Style Guide and
  COLPRAC.
• Any new documentation only uses public API

Additional context

Add any other context about the problem here.

[cstjean]

❤️

Would be nice to also revamp the Getting started with ModelingToolkit tutorial. Having f call an external f_fun which itself relies on a global variable was a big turn-off when I first looked into external data support.