Update pathlib tutorial

Based on my experience of reading previous notes, with respect to your opinion, I think The changes have created a better structure. Some changes have been made:

added " The Anatomy of a PosixPath" And merge it with "Picking Out Components of a Path" With a little change
added " Creating directories with pathlib" section with its examples
added " list of all files and directories" section with all its subcategories

The "Table of contents" section remains unchanged. if my changes are accepted please add them there.

pytopia / CS-Tutorial

Update pathlib tutorial #3