doc: add examples for permutations.jl

[inkydragon]

Add examples with doctest:

• permutations(a)
• permutations(a, t)
• multiset_permutations(a, t)
• nthperm!(a, k)
• nthperm(a, k)
• nthperm(p)
• levicivita(p)
• parity(p)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.88%. Comparing base (85c7193) to head (d933e95).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #173   +/-   ##
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  Coverage   96.88%   96.88%           
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  Files           7        7           
  Lines         803      803           
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  Hits          778      778           
  Misses         25       25           

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