Add SOS CKEditor configuration demo

README.md

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+# CKEditor 5 SOS demo
+
+Small CiviCRM extension demonstrating how a site-specific module can customize
+the main `ckeditor5` extension through its `crmCkeditor5Config` event.
+
+The demo mirrors the settings from `themes/sos_mjml`:
+
+- Roboto with Arial and sans-serif fallbacks;
+- email font sizes: 10, 11, 13, 14, 18 and 20 px;
+- SOS pink `#EA148C`;
+- SOS blue `#00577B`;
+- yellow `#FFCF1A`;
+- light blue `#2E78CC`;
+- white and black.
+
+The bundled Roboto files are copied unchanged from the theme. Font, size and
+color choices made through the toolbar are stored as inline CKEditor content so
+the email output does not depend on editor-only preview CSS.
+
+## Dependency
+
+This demo requires a `ckeditor5` build providing the `crmCkeditor5Config`
+event. CiviCRM currently checks that the extension is enabled, but does not
+enforce dependency versions declared in `info.xml`.