Add decimal column comparison operations

[isVoid]

Closes #7498

This PR adds binary comparison eq, lt, gt, le, ge to

• decimal column v. decimal column
• decimal column v. decimal scalar (decimal.Decimal and decimal cudf.Scalar)
• decimal column v. integer column (cudf.utils.dtypes.INTEGER_TYPES)
• decimal column v. integer scalar (Python ints)

Other minor adds:

• Supports binary ops between cudf.DecimalColumn and cudf.Scalar, where Scalar.dtype is cudf.Decimal64Dtype (follow up for [FEA] Implement DecimalColumn + Scalar and add cudf.Scalars of Decimal64Dtype #7732 )
• Short comment noting use of decimal64 in decimals.pxd
• Adding decimal data type in basics.rst

[codecov]

Codecov Report

Merging #7716 (f74b759) into branch-0.19 (7871e7a) will increase coverage by 0.80%.
The diff coverage is n/a.

❗ Current head f74b759 differs from pull request most recent head e8bfde2. Consider uploading reports for the commit e8bfde2 to get more accurate results

@@               Coverage Diff               @@
##           branch-0.19    #7716      +/-   ##
===============================================
+ Coverage        81.86%   82.67%   +0.80%     
===============================================
  Files              101      103       +2     
  Lines            16884    17563     +679     
===============================================
+ Hits             13822    14520     +698     
+ Misses            3062     3043      -19     

Impacted Files	Coverage Δ
python/cudf/cudf/utils/dtypes.py	83.54% <0.00%> (-5.97%)	⬇️
python/cudf/cudf/utils/gpu_utils.py	53.65% <0.00%> (-4.88%)	⬇️
python/cudf/cudf/core/column/lists.py	87.32% <0.00%> (-4.08%)	⬇️
python/cudf/cudf/core/column/decimal.py	92.68% <0.00%> (-2.19%)	⬇️
python/dask_cudf/dask_cudf/backends.py	87.50% <0.00%> (-2.13%)	⬇️
python/cudf/cudf/core/abc.py	87.23% <0.00%> (-1.14%)	⬇️
python/cudf/cudf/core/column/column.py	87.53% <0.00%> (-0.23%)	⬇️
python/cudf/cudf/utils/utils.py	85.36% <0.00%> (-0.07%)	⬇️
python/cudf/cudf/io/feather.py	100.00% <0.00%> (ø)
python/cudf/cudf/utils/ioutils.py	78.71% <0.00%> (ø)
... and 49 more

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[kkraus14]

@gpucibot merge