[INS-254] Create section for `pathlib` library and examples for all common usages.

[kevinamstutz]

[lsukova]

What parts of pathlib are you looking to include for this page? Obviously the example in the old book about file size is useful, I was just wondering how in-depth you want the page. Concrete paths, pure paths, or both? Include the differences between Windows and UNIX? Which methods do they need to know about? I've been using the documentation for reference, I'd be happy to feature everything if you have a list of what's needed