docs: add filter docstring examples to date and datetime

[marenwestermann]

contributes to #13161

I added examples for how to use polars.date and polars.datetime in combination with row filtering in a dataframe.

ping @MarcoGorelli

[MarcoGorelli]

Nice one!

Couple of suggestions:

• before the example you add, perhaps add an introductory sentence? Like "We can also use pl.datetime for filtering", just to distinguish it from the previous example
• it may be worth mentioning that pl.datetime allows you to overcome the standard library datetime's limitation of non supporting nanoseconds. Maybe the example could show this? You can't construct a nanosecond-precision datetime with datetime.datetime, but you can do it with strings, like

df = pl.DataFrame({'start': ['2020-01-01T01:02:03.123456789']})
df.with_columns(pl.col('start').str.to_datetime(time_unit='ns'))

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@@            Coverage Diff             @@
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+ Coverage   80.96%   80.98%   +0.01%     
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  Files        1384     1386       +2     
  Lines      178102   178382     +280     
  Branches     3043     3059      +16     
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- Misses      33426    33440      +14     
- Partials      482      488       +6     

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[MarcoGorelli]

should be good, thanks @marenwestermann !