Update robots.txt: Allow access to fonts

Author: tatsuya6502

robots.txt

@@ -1,8 +1,13 @@
 User-agent: *
+
 Disallow: /the-rust-programming-language-ja/1.6/
 Allow: /the-rust-programming-language-ja/1.6/book/
 Allow: /the-rust-programming-language-ja/1.6/rust.css
+Allow: /the-rust-programming-language-ja/1.6/*.woff
+
 Disallow: /the-rust-programming-language-ja/1.9/
 Allow: /the-rust-programming-language-ja/1.9/book/
 Allow: /the-rust-programming-language-ja/1.9/rust.css
+Allow: /the-rust-programming-language-ja/1.9/*.woff
+
 Sitemap: https://doc.rust-jp.rs/sitemap.xml