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

src/exercises/Cargo.toml

@@ -34,11 +34,11 @@ path = "day-3/simple-gui.rs"
 
 [[bin]]
 name = "dining-philosophers"
-path = "day-4/dining-philosophers.rs"
+path = "concurrency/dining-philosophers.rs"
 
 [[bin]]
 name = "link-checker"
-path = "day-4/link-checker.rs"
+path = "concurrency/link-checker.rs"
 
 [dependencies]
 reqwest = { version = "0.11.12", features = ["blocking"] }

src/exercises/android/morning.md

@@ -1,7 +1,7 @@
 # Exercises
 
-For the last exercise, we will look at one of the projects you work with. Let us
-group up and do this together. Some suggestions:
+This is a group exercise: We will look at one of the projects you work with and
+try to integrate some Rust into it. Some suggestions:
 
 * Call your AIDL service with a client written in Rust.
 

src/exercises/concurrency/solutions-morning.md

@@ -1,4 +1,4 @@
-# Day 4 Morning Exercise
+# Concurrency Morning Exercise
 
 ## Dining Philosophers
 

src/exercises/day-4/async-elevator.md

@@ -1 +0,0 @@
-# Exercises