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/Internal/Enumerations/StaticFileTests.swift

@@ -82,7 +82,7 @@ struct StaticFileTests {
     
     @Test
     func `all cases`() {
-        #expect(File.allCases.count == 7)
+        #expect(File.allCases.count == 8)
     }
     
 }
@@ -95,6 +95,7 @@ private extension StaticFileTests {
     
     static let contentTypes: [String] = [
         "text/javascript",
+        "text/css",
         "image/vnd.microsoft.icon",
         "image/png",
         "image/svg+xml",
@@ -104,6 +105,7 @@ private extension StaticFileTests {
     ]
     static let fileExtensions: [FileExtension] = [
         .js,
+        .css,
         .ico,
         .png,
         .svg,
@@ -113,6 +115,7 @@ private extension StaticFileTests {
     ]
     static let fileNames: [String] = [
         "app",
+        "error",
         "favicon",
         "icon",
         "icon",
@@ -122,6 +125,7 @@ private extension StaticFileTests {
     ]
     static let relativePaths: [String] = [
         "js/app.js",
+        "css/error.css",
         "favicon.ico",
         "icon.png",
         "icon.svg",