* Align outline with new spin-off course structure
With the new structure, the section on Android is a spin-off course
from the main 3-day course on Rust Fundamentals. The Bare-metal and
Concurrency days are spin-off courses in the same way.
* Explain new course structure
* Align Bare-Metal welcome page with other deep dives
* Merge Day 4 page into Course Structure page
* Remove Day 4 Welcome page
This aligns the Concurrency in Rust section with the Bare-Metal Rust
deep dive.
* Show subsections for Android deep dive
This aligns the Rust in Android section with the other deep dives.
* Clean up welcome page and README
We now cover async Rust and the course is no longer a four day course.
* Remove reference to the old Day 4
* Remove Day 4 references from exercises
