[REVIEW] Add dedicated page for StringHandling in python docs

[galipremsagar]

Resolves: #9246

This PR :

• Adds a dedicated page for StringHandling APIs.
• Fixes some of the StringMethods docstrings.
• Added some of the missing StringMethods APIs which were recently added.

[galipremsagar]

rerun tests

[codecov]

Codecov Report

Merging #9624 (fb1fb6a) into branch-21.12 (ab4bfaa) will decrease coverage by 0.12%.
The diff coverage is n/a.

@@               Coverage Diff                @@
##           branch-21.12    #9624      +/-   ##
================================================
- Coverage         10.79%   10.66%   -0.13%     
================================================
  Files               116      117       +1     
  Lines             18869    19825     +956     
================================================
+ Hits               2036     2115      +79     
- Misses            16833    17710     +877     

Impacted Files	Coverage Δ
python/dask_cudf/dask_cudf/sorting.py	92.90% <0.00%> (-1.21%)	⬇️
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)
... and 67 more

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[galipremsagar]

@gpucibot merge