Add tutorial for tabular data classification

[weijinglok]

This PR introduces the tabular data classification tutorial to the docs site.

This tutorial has been reviewed by @jwmueller, but may benefit if reviewed by other reviewers too.

[codecov]

Codecov Report

Merging #151 (d5dc556) into master (1247f16) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #151   +/-   ##
=======================================
  Coverage   87.64%   87.64%           
=======================================
  Files          12       12           
  Lines        1028     1028           
  Branches      191      191           
=======================================
  Hits          901      901           
  Misses        104      104           
  Partials       23       23           

Impacted Files	Coverage Δ
cleanlab/classification.py	96.00% <0.00%> (ø)

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[jwmueller]

Already did my edits offline; just make the one line change I recommended in the comments below before we merge this.

[jwmueller]

already edited offline