[no-verify] Integrate mdbook-linkcheck in book.toml and README.md.

Author: nate

README.md

@@ -18,6 +18,7 @@ There are two github CI workflows:
 
 - `cargo install mdbook`
 - `cargo install mdbook-graphviz`
+- `cargo install mdbook-linkcheck`
 
 ### Other prerequisites
 

book.toml

@@ -12,6 +12,8 @@ build-dir = "./docs"
 [output.html]
 default-theme = "light"
 
+[output.linkcheck]
+
 [preprocessor.graphviz]
 command = "mdbook-graphviz"
 after = ["links"]