Issue 5521 - Restructure CMS documentation to make it easier to use and extend

docs/structure/locales.md

Index: docs/structure/locales.md
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+# Localization files #
+
+The language-specific data is stored in the `locales` directory. Each language
+(identified by its locale code) is given a subdirectory here. The `.json` files
+contain localizable strings using the following format:
+
+    {
+      "stringid": {
+        "message": "Translated string",
+        "description": "Optional string description"
+      },
+      ...
+    }
+
+Any other files are considered localizable files and will be available on the
+server unchanged. This is useful for images for example: a language-dependent
+image can be placed into the `locales/en` directory and a German version
+of the same image into the `locales/de` directory. E.g. the file
+`locales/en/foo.png` will be available on the server under the URL `/en/foo.png`.