Move CSS files into theme/ folder (#1229)

Hi all,
This PR fixes https://github.com/google/comprehensive-rust/issues/1226:

* Moved CSS files to `theme/css`
* Moved speaker-notes.js to `theme/`
* Updated paths in book.toml

book.toml

@@ -29,12 +29,12 @@ class = "bob"
 
 [output.html]
 curly-quotes = true
-additional-js = ["speaker-notes.js"]
+additional-js = ["theme/speaker-notes.js"]
 additional-css = [
-  "svgbob.css",
-  "speaker-notes.css",
-  "language-picker.css",
-  "frontmatter.css",
+  "theme/css/svgbob.css",
+  "theme/css/speaker-notes.css",
+  "theme/css/language-picker.css",
+  "theme/css/frontmatter.css",
 ]
 site-url = "/comprehensive-rust/"
 git-repository-url = "https://github.com/google/comprehensive-rust"