docs: Shortest path example

[bobluppes]

• Adds example usage of shortest path algorithm introduced in feat: Unweighted shortest path #18
• Change signature of vertex and edge writers to accept vertex id and edge id as parameters

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3f8d83d) 99.84% compared to head (e3b5884) 99.84%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #25   +/-   ##
=======================================
  Coverage   99.84%   99.84%           
=======================================
  Files          15       15           
  Lines         651      651           
=======================================
  Hits          650      650           
  Misses          1        1           

Impacted Files	Coverage Δ
src/graaflib/io/dot.h	100.00% <100.00%> (ø)
src/graaflib/io/dot.tpp	100.00% <100.00%> (ø)
test/graaflib/io/dot_test.cpp	100.00% <100.00%> (ø)

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