Don't add a browser-cache busting hash to the generated files.
mdbook 0.5 made this the default behavior.

Index: docs/book/book.toml
--- docs/book/book.toml.orig
+++ docs/book/book.toml
@@ -12,6 +12,7 @@ edit-url-template = "https://github.com/rust-lang/rust
 git-repository-url = "https://github.com/rust-lang/rust-analyzer/tree/master/docs/book"
 mathjax-support = true
 site-url = "/book/"
+hash-files = false
 
 [output.html.playground]
 editable = true
@@ -33,3 +34,5 @@ use-boolean-and = true
 [output.html.fold]
 enable = true
 level = 3
+
+[output.markdown]
