Book intro chapters

[prrao87]

This PR adds the introduction piece's chapters to the book.

@sanders41 I'm leaning towards this general structure for each piece:

• Create chapters within each piece's directory, with each chapter covering a core concept
• This approach avoids cramming too much into one piece's written content, and avoids the reader having to "endlessly" scroll down to get to the end
• Add a "Takeaways" header at the end of each chapter to help the reader summarize the key ideas from that chapter

I'll continue adding more chapters over the coming days. I just wanted to get some of the book's content going before I go back to writing some code again :)

[sanders41]

It looks like the idea is to create the topic under pieces, then put each major concept for that piece in its own md file. Is that right?

I pulled the branck down and ran it and I think it looks good this way. I can work on doing this with the pieces I put in after we get this one merged.

[prrao87]

Yup, the idea is to not rigidly break each piece into Python and Rust sections in the book, as the point of it is to illustrate the thought process and what can be learned from the piece of code. That's why I felt it made sense to compare and contrast each snippet of code (rather than dumping the whole file's code into a single markdown block).

The GitHub readme shows well enough how to work with the code, so I didn't go into the minutiae of how to run the code, what steps to follow, etc. But at the start of each piece, it makes sense to link to the subdirectory on GitHub so the reader can optionally look at the code rightaway.

[prrao87]

Merging the first 4 md pages to get the correct doc structure in place in main. Will continue to add more this week.