Prepare for mdbook 0.5 migration

Because the pull_request checks are building docs from main, the checks
fail due to mdbook 0.5 changing and removing some fields. This change
should allow the books to work with both 0.4 and 0.5. In particular:

- `multilingual` -- This was never used, is optional, and is ignored in
  0.4. This is rejected in 0.5.
- `curly-quotes` -- This was renamed to `smart-punctuation` a long while
  ago. Both 0.4 and 0.5 use `smart-punctuation`, and 0.5 has removed the
  old `curly-quotes`.
- `src` -- This isn't needed and is an artifact of an `mdbook init` bug.

Author: ehuss

doc/dev-guide/book.toml

@@ -1,12 +1,10 @@
 [book]
 authors = ["The Rust Project Developers"]
 language = "en"
-multilingual = false
-src = "src"
 title = "The Rustup developer guide"
 
 [output.html]
-curly-quotes = true
+smart-punctuation = true
 edit-url-template = "https://github.com/rust-lang/rustup/edit/HEAD/doc/dev-guide/{path}"
 git-repository-url = "https://github.com/rust-lang/rustup/tree/HEAD/doc/dev-guide"
 site-url = "https://rust-lang.github.io/rustup/dev-guide"

doc/user-guide/book.toml

@@ -3,7 +3,7 @@ authors = ["The Rust Project Developers"]
 title = "The rustup book"
 
 [output.html]
-curly-quotes = true
+smart-punctuation = true
 edit-url-template = "https://github.com/rust-lang/rustup/edit/HEAD/doc/user-guide/{path}"
 git-repository-url = "https://github.com/rust-lang/rustup/tree/HEAD/doc/user-guide"
 site-url = "https://rust-lang.github.io/rustup/"