Cleanup references to "Day 4" (#603)

* 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

README.md

@@ -4,10 +4,14 @@
 [![GitHub contributors](https://img.shields.io/github/contributors/google/comprehensive-rust?style=flat-square)](https://github.com/google/comprehensive-rust/graphs/contributors)
 [![GitHub stars](https://img.shields.io/github/stars/google/comprehensive-rust?style=flat-square)](https://github.com/google/comprehensive-rust/stargazers)
 
-This repository has the source code for Comprehensive Rust 🦀, a four day Rust
+This repository has the source code for Comprehensive Rust 🦀, a multi-day Rust
 course developed by the Android team. The course covers all aspects of Rust,
-from basic syntax to generics and error handling. It also includes
-Android-specific content on the last day.
+from basic syntax to generics and error handling. It also includes deep dives on
+[Android], [bare-metal], and [concurrency].
+
+[Android]: https://google.github.io/comprehensive-rust/android.md
+[bare-metal]: https://google.github.io/comprehensive-rust/bare-metal.md
+[concurrency]: https://google.github.io/comprehensive-rust/concurrency.md
 
 Read the course at **https://google.github.io/comprehensive-rust/**.