HTML rendering support for the Website service (#5)

This PR contains the work done to replace the use of static _HTML_ files with type-safe HTML rendered server-side via **Elementary** through **Hummingbird**.

To provide further details about the work done:

* Added the **Elementary** dependencies.
* Added the `IndexPage` and `ErrorPage` pages, ported from the old HTML boilerplate; removed the static files.
* Added the `RootController` controller serving GET / using the `IndexPage` page, wired into the router.
* Reworked the `NotFoundMiddleware` middleare to render `ErrorPage` page directly; non-notFound errors still propagate.
* the `FileMiddleware` middleware no longer searches for any static `index.html` file.
* Simplified the `StaticFile` enumeration, dropped unused constants and now-unnecessary throws.

Reviewed-on: rock-n-code/loud-amsterdam#5
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Sources/Library/Public/Controllers/RootController.swift

@@ -0,0 +1,71 @@
+import Hummingbird
+import HummingbirdElementary
+
+/// Serves the website's root routes.
+///
+/// The controller exposes its routes as a `RouteCollection` so they can be added to a router
+/// (or a sub-group) by the application that composes it:
+///
+/// ```swift
+/// router.addRoutes(RootController<AppRequestContext>().routes)
+/// ```
+///
+/// - Note: `Context` is the request context the routes are resolved against, and must match the
+///   context of the router the routes are added to.
+public struct RootController<Context: RequestContext> : Sendable{
+
+    // MARK: Initializers
+
+    /// Creates a root controller.
+    public init() {}
+
+    // MARK: Properties
+
+    /// The routes served by the controller.
+    ///
+    /// Serves a `GET` request for the root path (`/`) by rendering the ``IndexPage``.
+    public var routes: RouteCollection<Context> {
+        let routes = RouteCollection(context: Context.self)
+
+        routes.get(
+            .Root.index,
+            use: index
+        )
+
+        return routes
+    }
+
+}
+
+// MARK: - Helpers
+
+private extension RootController {
+    
+    // MARK: Methods
+
+    /// Handles a request for the landing page.
+    /// - Parameters:
+    ///   - request: the incoming request.
+    ///   - context: the context the request is resolved against.
+    /// - Returns: an HTML response that renders the ``IndexPage``.
+    @Sendable
+    func index(
+        request: Request,
+        context: some RequestContext
+    ) -> HTMLResponse {
+        .init {
+            IndexPage()
+        }
+    }
+    
+}
+
+// MARK: - Constants
+
+extension RouterPath {
+    /// A namespace for the ``RootController`` route paths.
+    enum Root {
+        /// The path of the landing page.
+        static let index: RouterPath = "/"
+    }
+}