Add Canvas Extensions website page (#1900)

Generate extensions data, add the extensions listing route/navigation, and include install URL copy actions pinned to the build commit SHA.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

website/astro.config.mjs

@@ -57,6 +57,7 @@ export default defineConfig({
             { label: "Skills", link: "/skills/" },
             { label: "Hooks", link: "/hooks/" },
             { label: "Workflows", link: "/workflows/" },
+            { label: "Canvas Extensions", link: "/extensions/" },
             { label: "Plugins", link: "/plugins/" },
             { label: "Tools", link: "/tools/" },
             { label: "Contributors", link: "/contributors/" },