[Update Concept] functions, and complete function arguments

[bobahop]

Updating the functions concept to prepare for the functions concept exercise. This includes completing the function arguments concept, which is currently TODO. Function arguments will be lightly covered in functions, but a link to the function arguments concept will be provided, so something should be in there, I think.

[BethanyG]

@bobahop This is great! Thank you so much for revising this and moving it forward. 💙 I've left a bunch of little nits. The only "required" change I'd request is that we use REPL formatting for these small code examples. I'd also add something in here somewhere about Python being an OOP language, and so functions are objects. Beyond that, this looks really good.

[BethanyG]

🤦🏽‍♀️ I also forgot .. we probably need to mention scoping/namespacing in here somewhere......

NEVERMIND. I forgot this was the intro. I think this stuff is better in about.md anyways. 😄

[bobahop]

Thank you for REPLing the examples. If I had seen the About.md first I might have done that.

[bobahop]

@BethanyG , thank you for adding the default arguments section! I conflated them with keyword arguments, but they deserve their own section.

[BethanyG]

Ok. Some last suggestions. Beyond these, I think we need a better local -> nonlocal -> global code example progression, but I couldn't suggest one in the interface. But after that, this is good to go.

[BethanyG]

I am going to approve this so we can merge it and work on a cleaner copy. Points we still need to address:

1. We need to remove the code examples for global and nonlocal and redo them for clarity.
2. There may be some formatting issues that have crept in. If so, we'll need to address them in a follow-on PR.