Implemented phase 0 for the POC prototype in the Website service.

Services/Website/Sources/Library/Internal/Components/SiteLayout.swift

@@ -0,0 +1,53 @@
+import Elementary
+
+/// The frame a page wraps its content in: a skip link, the ``SiteHeader``, the content inside `main`, and the ``SiteFooter``.
+struct SiteLayout<Content: HTML>: HTML {
+
+    // MARK: Properties
+
+    /// The format marked as current in the header, or `nil` for none.
+    let active: Format?
+
+    /// The screen's markup, rendered inside the `main` landmark.
+    let content: Content
+
+    /// The signed-in viewer, or `nil` to render the signed-out header.
+    let viewer: Viewer?
+
+    // MARK: Initializers
+
+    /// Creates a site layout around a screen's content.
+    /// - Parameters:
+    ///   - active: the format marked as current, or `nil` (the default) for none.
+    ///   - viewer: the signed-in viewer, or `nil` (the default) to render the signed-out header.
+    ///   - content: the screen's markup.
+    init(
+        active: Format? = nil,
+        viewer: Viewer? = nil,
+        @HTMLBuilder content: () -> Content
+    ) {
+        self.active = active
+        self.content = content()
+        self.viewer = viewer
+    }
+
+    // MARK: Computed
+
+    var body: some HTML {
+        a(
+            .class("skip-link"),
+            .href("#main")
+        ) {
+            "Skip to content"
+        }
+        SiteHeader(
+            active: active,
+            viewer: viewer
+        )
+        main(.id("main")) {
+            content
+        }
+        SiteFooter()
+    }
+
+}