[WIP] Rust Extension Pages - Basics

[CrackTheCode016]

The following are proposed extension pages for capturing Rust in a Substrate context. These are created with assumed knowledge of Rust. The following topics are covered:

• Alternative Data Types
• Useful Traits - Common Substrate Traits
• Casting & Type Conversion
• Using Substrate Arithmetic for Safe Math

This is now divided into two main sections: Rust Basics and Rust Basics Extended. This is not definitive. It's simply a way to make it easier to read for now.

This PR also opens the floor to what I was thinking about structuring these pages, not just text, but somewhat interactive pieces of content. See issue: https://github.com/w3f/w3f-education/issues/111

• Include a notion of "quizzes", not checked in this case, but at least some sort of knowledge check as someone is reading.
• "Mini" summarizations of what is being discussed, especially when introducing Substrate into Rust like this
• Runnable code would be awesome. I'm considering using replit for that, aka solving https://github.com/w3f/w3f-education/issues/142

[CrackTheCode016]

@DrW3RK This is turning into quite a long page - could you please take a look, and maybe I split this into separate PRs (and make sure it reads well)?