Adjustments to the Localization package dependency (#11)

This PR contains the work done to refactor the _Localization_ package and tidy the project a little bit.

To provider further details about the work:

* The `Negotiate` function was moved to the _Localization_ package.
* The `WebsiteRequestContext` context no longer resolves a default language at creation; it starts empty and relies on the `LocalizationMiddleware` middleware to fill it in.
* Fixed the local dependency path to Packages/Localization for the **Website** package as it only resolved inside the Xcode workspace before, breaking swift build, the Makefile, and the Docker build).
* Updated the `README` file to document language negotiation, the GET /health route, and the full middleware chain; doc comments across the moved/renamed types were brought back in sync with the code.

Reviewed-on: rock-n-code/loud-amsterdam#11
Co-authored-by: Javier Cicchelli <javier@rock-n-code.com>
Co-committed-by: Javier Cicchelli <javier@rock-n-code.com>

Services/Website/Package.swift

@@ -1,4 +1,4 @@
-// swift-tools-version:6.3
+// swift-tools-version: 6.3
 
 import PackageDescription
 
@@ -21,7 +21,7 @@ let package = Package(
     ],
     dependencies: [
         .package(
-            path: "../Packages/Localization"
+            path: "../../Packages/Localization"
         ),
         .package(
             url: "https://github.com/elementary-swift/elementary.git",