docs(volunteer): add 'aanmelden via een link' page

Explains the public-link registration flow for vrijwilligers and wires
the page into the VitePress sidebar.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

docs/.vitepress/config.ts

@@ -131,6 +131,7 @@ export default defineConfig({
           items: [
             { text: "Overzicht", link: "/volunteer/" },
             { text: "Registreren", link: "/volunteer/registration" },
+            { text: "Je aanmelden via een link", link: "/volunteer/je-aanmelden-via-een-link" },
             { text: "Diensten kiezen", link: "/volunteer/shift-signup" },
             { text: "Mijn paspoort", link: "/volunteer/passport" },
             { text: "Check-in", link: "/volunteer/check-in" },