Add topological_sort_by_dfs to docs on traversals

[gdalle]

Solves #1566

[codecov]

Codecov Report

Merging #1588 (712b8d1) into master (1477bbc) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1588   +/-   ##
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  Coverage   99.44%   99.44%           
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  Files         106      106           
  Lines        5551     5551           
=======================================
  Hits         5520     5520           
  Misses         31       31           

[gdalle]

Thanks for the merge @sbromberger. Do you think I could become part of the JuliaGraphs organization? I would like to contribute to a handful of packages related to graphs, and possibly tutor student projects on that topic.