Document use of Distributions with eFAST

[scheidan]

Documents the possibility to use Distributions to define input ranges for eFAST. This was enabled with by
#133

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.

[ChrisRackauckas]

Should it be a separate example?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 92.28%. Comparing base (58f0c5e) to head (b850cd0).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #163      +/-   ##
==========================================
- Coverage   93.01%   92.28%   -0.73%     
==========================================
  Files          11       11              
  Lines         830      830              
==========================================
- Hits          772      766       -6     
- Misses         58       64       +6     

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[Vaibhavdixit02]

A separate example might be nice if @scheidan can put one together, otherwise, this looks fine as well since it clearly demonstrates the use of distributions different from the ranges (and this is just the docstring)