Upgrade @metalsmith/layouts to version 3.0.0

This is a major version bump, and the changelog at
https://github.com/metalsmith/layouts/releases/tag/v3.0.0
indicates a breaking change that impacts us, namely that we need to
explicitly define the pattern and transformer that we wish to use.

gulpfile.mjs

@@ -2266,6 +2266,8 @@ gulp.task("metalsmith", async function () {
       .use(
         layouts({
           directory: "docs/templates",
+          pattern: "**",
+          transform: "nunjucks",
         })
       )
       .use(relative())