Optimizations for the Website service (#9)

This PR contains the work done to provide optimizations to the current service, such as a health-check endpoint, pre-renders static HTML pages, and hardens the error page's CSP.

To provide further details about the work:

* Added the `HealthController` controller serving GET `/health` with a static JSON payload.
* Added the `CachedHTMLResponse` response, which renders a static HTMLDocument to bytes once and reuses them per request (no Content-Length, so responses stay compressible).
* Integrated the response into the `RootController` and the  `NotFoundMiddleware` middleware to avoid re-rendering on hot paths.
* Added a `RouterMethods.addRoutes(_:)` extension and switched the router in App+build to use it.
* Moved the inline style from the `ErrorPage` page into a dedicated style file so the CSP needs no inline-style escape hatch.
* Fixed the `IndexPage` page path inconsistencies.
* Written the `README` file.

Reviewed-on: rock-n-code/loud-amsterdam#9
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Tests/Library/Cases/Public/Controllers/HealthControllerTests.swift

@@ -0,0 +1,39 @@
+import Hummingbird
+import HummingbirdTesting
+import NIOCore
+import Testing
+
+@testable import WebsiteCore
+
+@Suite("HealthController controller")
+struct HealthControllerTests {
+
+    // MARK: Constants
+
+    private let app: Application = .init(router: {
+        let router = Router()
+
+        router.addRoutes(HealthController<BasicRequestContext>().routes)
+
+        return router
+    }())
+
+    // MARK: Functional tests
+
+    @Test
+    func `serves the status payload at the health path`() async throws {
+        try await app.test(.router) { client in
+            try await client.execute(
+                uri: "/health",
+                method: .get
+            ) { response in
+                let body = String(buffer: response.body)
+
+                #expect(response.status == .ok)
+                #expect(response.headers[.contentType] == "application/json")
+                #expect(body == #"{"status":"ok"}"#)
+            }
+        }
+    }
+
+}