Merge pull request #1952 from HollowMan6/master

Support building books using mdbook in main (v0.5.x)

Author: marioidival

book.toml

@@ -5,7 +5,6 @@ authors = ["The Rust Community"]
 
 [output.html.playpen]
 editable = true
-editor = "ace"
 line-numbers = true
 
 [output.html.fold]
@@ -27,4 +26,5 @@ edition = "2021"
 extra-watch-dirs = ["po"]
 
 [preprocessor.gettext]
+optional = true
 after = ["links"]