Propose additions to top-level README and new project structure

[evazorro]

Add statement about intent of docs, pull some overview content into this top-level README, then add a section directing readers to different parts of the docs based on what they're trying to accomplish.

Also include a (fake) table of contents with my proposed restructuring of the documentation. Notes are given in "//" comments and reference existing content in relative file paths where appropriate.

[kytrinyx]

@evazorro Just a quick note to say that this is super high on my list, but that I'm wrestling some bugs in the CLI, which is affecting a bunch of people. I expect to be able to tackle this properly starting September 1, at which point it will be my primary focus (aside from the day-job, at least).

[mbbroberg]

Hi @evazorro! I'm helping review this PR while I consult with the Exercism team. I'm really excited to dive in - thank you for making it so easy to follow along with the notes 😄

I'll get back to you soon!