fix(manual): disable linkcheck to fix deploy

manual/book.toml

@@ -15,6 +15,7 @@ git-repository-icon = "fa-github"
 # Should we check links on the internet? Enabling this option adds a
 # non-negligible performance impact
 follow-web-links = false
+optional = true
 
 # Are we allowed to link to files outside of the book's root directory? This
 # may help prevent linking to sensitive files (e.g. "../../../../etc/shadow")