Intro to os module

[izaakm]

Addresses issue #554.

Changes:

• Remove markdown content from Episode 1.
• Add content about the os library to Episode 6, specifically, introduce the getcwd, listdir, and chdir methods.

[alee]

Thanks for the PR! I'm inclined to move the markdown removal into a separate PR so others in the community can chime in on whether they think it's worth removing.

We could also mark it as optional so instructors know to assess their learners when they are teaching and can skip over it if most people know / understand how it works.

Thoughts from others?

[martinosorb]

I'd just like to point out that Pathlib is a new library part of Python's core libraries, and is now considered a much better standard for filesystem operations. Path.cwd(), Path("...").iterdir() can do the same as os.getcwd() and os.listdir(). As for chdir, I personally don't use it and think changing wd tends to cause trouble.