[REVIW] Miscellaneous documentation fixes to cudf

[galipremsagar]

This PR:

• Exposes .struct accessor
• Exposes Decimal32Dtype
• Adds newly introduced Datetime related APIs
• Fixes some examples.

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Codecov Report

Merging #9471 (4411060) into branch-21.12 (ab4bfaa) will decrease coverage by 0.13%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9471      +/-   ##
================================================
- Coverage         10.79%   10.65%   -0.14%     
================================================
  Files               116      117       +1     
  Lines             18869    19760     +891     
================================================
+ Hits               2036     2106      +70     
- Misses            16833    17654     +821     

Impacted Files	Coverage Δ
python/cudf/cudf/io/csv.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/hdf.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/orc.py	0.00% <0.00%> (ø)
python/cudf/cudf/__init__.py	0.00% <0.00%> (ø)
python/cudf/cudf/_version.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/abc.py	0.00% <0.00%> (ø)
python/cudf/cudf/api/types.py	0.00% <0.00%> (ø)
python/cudf/cudf/io/dlpack.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/frame.py	0.00% <0.00%> (ø)
python/cudf/cudf/core/index.py	0.00% <0.00%> (ø)
... and 61 more

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[isVoid]

I recently also noticed that the docstrings of rand_dataframe in dataset_generator.py is lacking description to list dtype dtype_meta. Specifically:

• lists_max_length
• nesting_max_depth
• value_type

Can you add them to this PR also as part of the doc fixes?

[galipremsagar]

I recently also noticed that the docstrings of rand_dataframe in dataset_generator.py is lacking description to list dtype dtype_meta. Specifically:

• lists_max_length
• nesting_max_depth
• value_type

Can you add them to this PR also as part of the doc fixes?

I think I'll need to be updating the docstrings of rand_dataframe of dataset_generator anyways in this PR: #9180, where I'm introducing struct support. So will tackle it 9180 instead of this PR.

[galipremsagar]

@gpucibot merge