(docs): Reordered and moved usage guide into an mdbook

[maddeleine]

Resolved issues:

Resolves #4040. Takes what #4277 started and attempts to break it down into a smaller change.

Description of changes:

This change takes our usage guide and splits each section into a separate file. I broke up the file in a way that fits into the rust mdbook format. Therefore, in order to review this change, you will have to check out my branch, install rust mdbook, and then run the mdbook serve command, which allows you to view the book on your localhost. This enables you to see how the guide has been reordered.

Alternatively, I can apply my reorganization to the usage guide as it is now (a large markdown file), if this change is too large. However, we're going to have to separate the sections into md files eventually, so I don't think this is that far off from where we want to end up.

Call-outs:

The changes I made to the content are listed here:

• I added an introduction section since the guide looked weird without it
• I moved a paragraph about consuming our library to the BUILD.md file as it didn't seem to fit in with any section.
• I applied consistent formatting to the way we style function names in markdown.
• I edited links so that they don't break for this change. I also checked that the internal links are working with mdbook linkchecker, which is so useful <3.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[maddeleine]

I realized that everything that links to the original usage guide in this repo will be broken by this change. Went through and updated all the links I could find that referenced the usage guide. I had to take some artistic liberties with the links as it seems like we do both relative links and absolute links.

So thats why I re-requested reviews 😄.