docs: add Handler doc

[vuongxuongminh]

No description provided.

[codecov-commenter]

Codecov Report

Merging #34 (72dffe0) into main (5105f51) will increase coverage by 0.15%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main      #34      +/-   ##
==========================================
+ Coverage   74.70%   74.85%   +0.15%     
==========================================
  Files          24       24              
  Lines        1933     1933              
==========================================
+ Hits         1444     1447       +3     
+ Misses        374      372       -2     
+ Partials      115      114       -1     

Impacted Files	Coverage Δ
handler.go	76.13% <ø> (ø)
caching_result.go	89.41% <0.00%> (ø)
schema_fetcher.go	85.65% <0.00%> (+1.30%)	⬆️

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