docs: fix several doc issues and missing API documentation

[thatlittleboy]

Port of #2020, with some notable changes:

• I didn't expose the Gradient and Kernel explainers to the public API, I think that is a bigger decision that warrants some level of discussion outside the scope of documentation changes.
• I didn't expose all of the shap.utils functions in our public API. E.g. shap.utils.safe_isinstance or shap.utils.assert_import are not very useful for others.
• We used to have two benchmark(s) folder in the docs, I merged the two together in this PR.
• Some minor fixes to the readthedocs config to resolve some warnings.

[codecov[bot]]

Codecov Report

Merging #102 (02df738) into master (b1252d9) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #102   +/-   ##
=======================================
  Coverage   53.58%   53.58%           
=======================================
  Files          90       90           
  Lines       12893    12893           
=======================================
  Hits         6909     6909           
  Misses       5984     5984           

Impacted Files	Coverage Δ
shap/explainers/__init__.py	100.00% <100.00%> (ø)
shap/utils/__init__.py	100.00% <100.00%> (ø)

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