[DOCS] Shorten union of Literals

[desmondcheongzx]

Small clean up to simplify docs by banning unnecessary union on literals.

For example, in DataFrame.write_parquet, the write_mode argument has the following signature:

write_mode: Union[Literal['append'], Literal['overwrite']] = 'append',

But according to PEP 586 --- Literal Types, this can be equivalently written as

write_mode: Literal['append', 'overwrite'] = 'append',

which is much easier for users to follow.

[codspeed-hq]

CodSpeed Performance Report

Merging #3449 will improve performances by 40.17%

_{Comparing desmondcheongzx:shorten-union-of-literals (0ce9a5d) with main (f2d4f73)}

Summary

⚡ 1 improvements
✅ 16 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	desmondcheongzx:shorten-union-of-literals	Change
⚡	test_iter_rows_first_row[100 Small Files]	304.2 ms	217 ms	+40.17%

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.19%. Comparing base (f2d4f73) to head (0ce9a5d).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3449      +/-   ##
==========================================
- Coverage   77.50%   77.19%   -0.31%     
==========================================
  Files         687      686       -1     
  Lines       84472    84834     +362     
==========================================
+ Hits        65467    65489      +22     
- Misses      19005    19345     +340     

Files with missing lines	Coverage Δ
daft/context.py	90.85% <100.00%> (ø)
daft/dataframe/dataframe.py	86.89% <ø> (ø)
daft/expressions/expressions.py	93.28% <100.00%> (ø)
daft/filesystem.py	71.11% <100.00%> (ø)
daft/runners/runner.py	79.31% <100.00%> (ø)
daft/series.py	89.55% <ø> (ø)
daft/table/table.py	58.16% <ø> (ø)
daft/udf_library/url_udfs.py	97.72% <100.00%> (ø)

... and 17 files with indirect coverage changes