Doc: clarify when to use pathlib pure paths [skip issue]

[YASWANTH1976]

Added explanation of when pure paths are appropriate to help beginners understand the difference between pure and concrete paths.

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📚 Documentation preview 📚: https://cpython-previews--143319.org.readthedocs.build/

[python-cla-bot]

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[da-woods]

This won't be indented correctly and has some formatting issues such as missing spaces.

I personally don't think it adds much that isn't already covered by the list of points (but that's really for someone else to decide - I have no say here)

[picnixz]

I don't think it's useful. We already say:

#. You want to make sure that your code only manipulates paths without actually
accessing the OS. In this case, instantiating one of the pure classes may be
useful since those simply don't have any OS-accessing operations.

I'm going to close this but if Barney really wants this addition, he can re-open this.

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