DOC: fix these doc doesn't exist in dtoolkit

[Zeroto521]

click source code link which exists in another package would get 404

[codecov]

Codecov Report

Merging #244 (5577b8c) into master (d45c3f0) will decrease coverage by 0.97%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #244      +/-   ##
==========================================
- Coverage   99.80%   98.82%   -0.98%     
==========================================
  Files          21       21              
  Lines         512      512              
  Branches       51       51              
==========================================
- Hits          511      506       -5     
- Misses          0        3       +3     
- Partials        1        3       +2     

Impacted Files	Coverage Δ
dtoolkit/accessor/_util.py	80.00% <0.00%> (-20.00%)	⬇️
dtoolkit/accessor/dataframe.py	97.14% <0.00%> (-1.43%)	⬇️

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