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/Sources/Library/Internal/Pages/IndexPage.swift

@@ -8,7 +8,7 @@ struct IndexPage: HTMLDocument, Sendable {
     /// The page's content.
     var body: some HTML {
         p { "Hello world! This is HTML5 Boilerplate." }
-        script(.src("js/app.js")) {}
+        script(.src("/js/app.js")) {}
     }
 
     /// The metadata, stylesheet, icon, and manifest links placed in the document head.
@@ -20,7 +20,7 @@ struct IndexPage: HTMLDocument, Sendable {
         )
         link(
             .rel(.stylesheet),
-            .href("css/style.css")
+            .href("/css/style.css")
         )
         link(
             .rel(.icon),
@@ -40,11 +40,11 @@ struct IndexPage: HTMLDocument, Sendable {
         )
         link(
             .rel("apple-touch-icon"),
-            .href("icon.png")
+            .href("/icon.png")
         )
         link(
             .rel("manifest"),
-            .href("site.webmanifest")
+            .href("/site.webmanifest")
         )
         meta(
             .name("theme-color"),
@@ -58,6 +58,8 @@ struct IndexPage: HTMLDocument, Sendable {
     }
 
     /// The document title.
-    var title: String { "" }
+    var title: String {
+        "Index page"
+    }
 
 }