Rename welcome.md to index.md (#1039)

When building the book, mdBook will always generate an `index.html`
page for first page of the book. This meant that we had the same
content available under two different names:

- `welcome.html`: this is what the TOC would link to, and
- `index.html` or simply `/`: this is what search engines link to

Renaming the page and setting up a redirect should fix this confusion.
We still don’t have a good way of avoiding links to the `index.html`
page, but this should fix the first half of the problem.

I tested this for translations as well by building the output in a
subdirectory and serving the parent directory.

Part of #847.

book.toml

@@ -72,6 +72,8 @@ line-numbers = true
 "unsafe/unsafe-functions.html" = "calling-unsafe-functions.html"
 "welcome-bare-metal.html" = "bare-metal.html"
 "welcome-day-4.html" = "concurrency.html"
+# Send people to canonical URL instead of index.html
+"welcome.html" = "./"
 
 [output.exerciser]
 output-directory = "comprehensive-rust-exercises"

src/SUMMARY.md

@@ -1,6 +1,7 @@
 # Summary
 
-[Welcome to Comprehensive Rust 🦀](welcome.md)
+<!-- Keep first page as index.md to avoid giving it two names. -->
+[Welcome to Comprehensive Rust 🦀](index.md)
 - [Running the Course](running-the-course.md)
   - [Course Structure](running-the-course/course-structure.md)
   - [Keyboard Shortcuts](running-the-course/keyboard-shortcuts.md)