[REVIEW] Documentation enhancements to cudf python APIs

[galipremsagar]

Fixes: #5210
Fixes: #3075
Fixes: #2681
Fixes: #5143

1. Added .str support for Index.
2. Fixed issue where calling .cat.codes is not assigning an index to the series.

>>> s
a    1
b    2
c    3
dtype: category
Categories (3, int64): [1, 2, 3]
>>> s.cat.codes
0    0
1    1
2    2
dtype: int8
>>> s.to_pandas().cat.codes
a    0
b    1
c    2
dtype: int8

3. Order of the resulting columns was wrong in applying math operation in data frame:

>>> import cudf
>>> df = cudf.DataFrame({'angles': [0, 3, 4],'degrees': [360, 180, 360]},index=['circle', 'triangle', 'rectangle'])
>>> other = cudf.DataFrame({'angles': [0, 3, 4]},index=['circle', 'triangle', 'rectangle'])
>>> df * other
          degrees  angles
circle       null       0
triangle     null       9
rectangle    null      16
>>> df.to_pandas() * other.to_pandas()
           angles  degrees
circle          0      NaN
triangle        9      NaN
rectangle      16      NaN

Fixed this issue.

4. Added rdiv, an alias to rtruediv
5. Index.argsort was returning a NumericalColumn, pandas returns a numpy array. Fixed this by making it return cupy array.
6. Index.to_series should actually return a series of values and index of itself to the series, fixed this issue as well.

>>> import pandas as pd
>>> s = pd.Index(['a','b','c'])
>>> s
Index(['a', 'b', 'c'], dtype='object')
>>> s.to_series()
a    a
b    b
c    c
dtype: object
>>> ds = cudf.from_pandas(s)
>>> ds
StringIndex(['a' 'b' 'c'], dtype='object')
>>> ds.to_series()
0    a
1    b
2    c
dtype: object

7. Exposing start & stop properties for RangeIndex.
8. Series.copy seems to be creating a copy for index in case of a deep copy, fixed this issue too: https://pandas.pydata.org/pandas-docs/stable/reference/api/pandas.Series.copy.html
9. Removed unique_k(deprecated 3 years back) and it’s associated dask_cudf methods.
10. Added Click to Copy & Click to remove prompt and output(picked from python docs:
    

[GPUtester]

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[galipremsagar]

rerun tests