Add docs examples for dynamically compare functions

[HaoYang670]

Signed-off-by: remzi [email protected]

Which issue does this PR close?

Closes #1202.

Rationale for this change

Make it easier for users to understand and use dyn compare functions.

What changes are included in this PR?

Are there any user-facing changes?

[codecov-commenter]

Codecov Report

Merging #1250 (bd29a98) into master (02573e9) will increase coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1250      +/-   ##
==========================================
+ Coverage   82.95%   82.96%   +0.01%     
==========================================
  Files         178      178              
  Lines       51522    51547      +25     
==========================================
+ Hits        42740    42766      +26     
+ Misses       8782     8781       -1     

Impacted Files	Coverage Δ
arrow/src/compute/kernels/comparison.rs	91.70% <ø> (ø)
arrow/src/array/transform/mod.rs	84.64% <0.00%> (-0.13%)	⬇️
parquet/src/arrow/array_reader.rs	78.21% <0.00%> (+0.01%)	⬆️
parquet_derive/src/parquet_field.rs	66.21% <0.00%> (+0.22%)	⬆️
parquet/src/arrow/arrow_reader.rs	93.57% <0.00%> (+0.38%)	⬆️
arrow/src/datatypes/datatype.rs	66.80% <0.00%> (+0.42%)	⬆️

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[alamb]

Thank you @HaoYang670 -- looks great