examples & docs: add examples and docs for Aggregate and Assign

[flycash]

No description provided.

[codecov]

Codecov Report

Merging #50 (71867f0) into main (2ae784b) will increase coverage by 0.40%.
The diff coverage is 76.19%.

@@            Coverage Diff             @@
##             main      #50      +/-   ##
==========================================
+ Coverage   80.74%   81.14%   +0.40%     
==========================================
  Files          14       14              
  Lines         748      748              
==========================================
+ Hits          604      607       +3     
+ Misses        122      120       -2     
+ Partials       22       21       -1     

Impacted Files	Coverage Δ
aggregate.go	95.71% <ø> (ø)
assignment.go	66.66% <0.00%> (ø)
expression.go	51.85% <50.00%> (+13.75%)	⬆️
builder.go	72.09% <86.66%> (ø)
column.go	60.56% <100.00%> (-3.08%)	⬇️
model.go	92.10% <0.00%> (+7.89%)	⬆️

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