Add type hierarchy display

[ablaom]

This PR exposes the same functionality introduced at JuliaAI/ScientificTypesBase.jl#30. It must be explicitly added, because ScientficTypes.scitype and ScientificTypesBase.scitype are not the same function.

This PR also includes some updates to the Documenter manual.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.88%. Comparing base (086ee0b) to head (c05de52).
⚠️ Report is 10 commits behind head on dev.

Additional details and impacted files

@@            Coverage Diff             @@
##              dev     #210      +/-   ##
==========================================
+ Coverage   92.87%   92.88%   +0.01%     
==========================================
  Files           6        6              
  Lines         435      436       +1     
==========================================
+ Hits          404      405       +1     
  Misses         31       31              

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