docs: write SymbolCache tutorial

[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.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f407c15) 24.85% compared to head (22d7ea1) 33.33%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   24.85%   33.33%   +8.47%     
==========================================
  Files           6        6              
  Lines         177      177              
==========================================
+ Hits           44       59      +15     
+ Misses        133      118      -15     

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[AayushSabharwal]

Oops, clicked what I thought was the button to delete the branch (because it doesn't do it by default on forks, and I forgot this isn't a fork 😓 )