Add spaces at the end of doc side nav toc

[weijinglok]

This PR adds spaces after the side navigation TOC in the doc site. A mock-up is available here.

[codecov]

Codecov Report

Merging #213 (4830d9c) into master (72a62ed) will decrease coverage by 0.07%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #213      +/-   ##
==========================================
- Coverage   95.48%   95.40%   -0.08%     
==========================================
  Files          12       12              
  Lines         909      915       +6     
  Branches      180      184       +4     
==========================================
+ Hits          868      873       +5     
- Misses         15       16       +1     
  Partials       26       26              

Impacted Files	Coverage Δ
cleanlab/dataset.py	89.18% <0.00%> (-0.52%)	⬇️

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[jwmueller]

LGTM thanks!