Restructure content

[hdoordt]

This PR is about restructuring the content, as well as making it more modular.

Using DocDoc, we're able to split up modules into smaller topic parts, each of which can be imported from some entry markdown file at will. This way, we can make Rust 101 much more suitable for reuse.

This PR furthermore implements changes discussed in #55.

TODO:

• Setup module A-foundations.
• Setup module B-crate-engineering
• Setup module C-concurrency
• Setup module D-rust-for-web
• Setup module E-rust-for-systems
• Setup module G-rust-for-data-science
• Update contributing guide
• Remove old content
• Update README to contain info on suggested tracks
• List learning objectives per topic
• Relate exercises to topics

Note: this PR will not add new content. Instead, it will add stubs for content that is to be created, calling for participation.