Add pivot_table and crosstab to docs.

[bdice]

Description

This PR resolves #12012 by adding cudf.pivot_table and cudf.crosstab to the documentation.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

Pandas references:
https://pandas.pydata.org/docs/reference/api/pandas.pivot_table.html
https://pandas.pydata.org/docs/reference/api/pandas.crosstab.html

[shwina]

@gpucibot merge

[codecov]

Codecov Report

Base: 87.40% // Head: 86.90% // Decreases project coverage by -0.49% ⚠️

Coverage data is based on head (ac3975f) compared to base (f72c4ce).
Patch has no changes to coverable lines.

❗ Current head ac3975f differs from pull request most recent head 7d3531e. Consider uploading reports for the commit 7d3531e to get more accurate results

Additional details and impacted files

@@               Coverage Diff                @@
##           branch-22.12   #12014      +/-   ##
================================================
- Coverage         87.40%   86.90%   -0.50%     
================================================
  Files               133      133              
  Lines             21833    21977     +144     
================================================
+ Hits              19084    19100      +16     
- Misses             2749     2877     +128     

Impacted Files	Coverage Δ
python/strings_udf/strings_udf/lowering.py	0.00% <0.00%> (-84.40%)	⬇️
python/strings_udf/strings_udf/_typing.py	81.05% <0.00%> (-14.74%)	⬇️
python/strings_udf/strings_udf/__init__.py	84.31% <0.00%> (-12.57%)	⬇️
python/cudf/cudf/io/text.py	91.66% <0.00%> (-8.34%)	⬇️
python/cudf/cudf/core/_base_index.py	82.20% <0.00%> (-3.35%)	⬇️
python/cudf/cudf/core/column/lists.py	92.78% <0.00%> (-0.97%)	⬇️
python/cudf/cudf/testing/dataset_generator.py	72.83% <0.00%> (-0.42%)	⬇️
python/dask_cudf/dask_cudf/core.py	73.72% <0.00%> (-0.41%)	⬇️
python/dask_cudf/dask_cudf/backends.py	84.90% <0.00%> (-0.37%)	⬇️
python/cudf/cudf/core/tools/datetimes.py	84.19% <0.00%> (-0.31%)	⬇️
... and 23 more

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