flake: Use lychee for link checking — rips

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.

       devShells.default = pkgs.mkShell {
         buildInputs = [
           pkgs.asciidoctor
           pkgs.lychee
         ];
       };
       checks.links = pkgs.testers.lycheeLinkCheck {
         site = pkgs.lib.sources.sourceFilesBySuffices ./. [".md" ".adoc"];
         extraConfig = {
           extensions = ["md" "adoc"];
           include_fragments = true;
           exclude = [
             "^rad:.*$"
             "^web\\+rad:.*$"
             "^vscode:.*$"
           ];
         };
       };
     });
 }

 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