flake: Use lychee for link checking

Introduce [lychee] link checking in the flake.nix checks.
Since `nix flake check` is limited by sandboxing, it only checks for
internal links.

Running `lychee` from the directory will check external links.
In the future, this can be done via git hooks.

[lychee]: https://github.com/lycheeverse/lychee

.gitignore

@@ -1,2 +1,4 @@
 node_modules/
 result
+.lycheecache
+.direnv/

flake.nix

@@ -26,6 +26,7 @@
         buildInputs =
           (with pkgs; [
             asciidoctor
+            pkgs.lychee
             nodejs
             shellcheck
             yq
@@ -35,20 +36,35 @@
           ];
       };
 
-      checks.grammar =
+      checks = {
-        pkgs.runCommand "grammar-check" {
+        grammar =
-          nativeBuildInputs =
+          pkgs.runCommand "grammar-check" {
-            (with pkgs; [
+            nativeBuildInputs =
-              nearley
+              (with pkgs; [
-              nodejs
+                nearley
-              yq
+                nodejs
-            ])
+                yq
-            ++ [
+              ])
-              nearley
+              ++ [
+                nearley
+              ];
+          } ''
+            ${./0004-general-uri-scheme/data/test/uri-scheme-tester.sh} ${./0004-general-uri-scheme/data}
+            touch $out
+          '';
+
+        links = pkgs.testers.lycheeLinkCheck {
+          site = pkgs.lib.sources.sourceFilesBySuffices ./. [".md" ".adoc"];
+          extraConfig = {
+            extensions = ["md" "adoc"];
+            include_fragments = true;
+            exclude = [
+              "^rad:.*$"
+              "^web\\+rad:.*$"
+              "^vscode:.*$"
             ];
-        } ''
+          };
-          ${./0004-general-uri-scheme/data/test/uri-scheme-tester.sh} ${./0004-general-uri-scheme/data}
+        };
-          touch $out
+      };
-        '';
     });
 }

lychee.toml

@@ -0,0 +1,12 @@
+extensions = ["md", "adoc"]
+include_fragments = true
+# Exclude custom URI schemes that lychee cannot resolve
+exclude = [
+  '^rad:',
+  '^web\+rad:',
+  '^vscode:',
+]
+include_mail = false
+timeout = 20
+max_retries = 2
+retry_wait_time = 2