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/running-the-course.md

@@ -17,7 +17,7 @@ Before you run the course, you will want to:
    the audience you expect, or on your own expertise.
 
 1. Decide on the dates. Since the course is large, we recommend that you
-   schedule the four days over two weeks. Course participants have said that
+   schedule the days over two weeks. Course participants have said that
    they find it helpful to have a gap in the course since it helps them process
    all the information we give them.
 
@@ -41,8 +41,6 @@ Before you run the course, you will want to:
    and offer a solution, e.g., by showing people where to find the relevant
    information in the standard library.
 
-1. Prepare anything you need to have available for the afternoon of day 4.
-
 That is all, good luck running the course! We hope it will be as much fun for
 you as it has been for us!