Document onboarding test URL parameters

Author: noisysocks

special-pages/pages/onboarding/readme.md

@@ -36,3 +36,17 @@ Don't edit the generated files directly - any changes you make will not be refle
 
 Instead, make your changes in `src/' and then run `npm run build.pages` from the `packages/special-pages` folder
 
+### Test URL params
+
+URL parameters can be used to override the default values of the onboarding flow's configuration. Supported parameters are:
+
+-   `locale` - Specifies the language to use for translations. Expects a language code (e.g., `en`, `de`, `fr`).
+
+- `locale` - Specifies the language to use for translations. Expects a language code (e.g., `en`, `de`, `fr`).
+- `textLength` - Adjusts the length of translated strings, for testing layout with different text sizes (e.g., `short`, `long`).
+- `display` - Controls which view is rendered. `app` renders the main onboarding flow, `components` renders the component showcase page.
+- `platform` - Overrides the detected platform, for testing platform-specific styling (e.g., `macos`, `windows`).
+- `order` - Specifies which predefined set of onboarding steps to use (e.g., `v1`, `v2`, `v3`).
+- `exclude` - Removes specific steps from the onboarding flow. Can be used multiple times (e.g., `&exclude=welcome&exclude=systemSettings`).
+- `page` - Sets the initial step to start the onboarding flow on (e.g., `systemSettings`).
+- `adBlocking` - If set to `enabled`, adds a step for enabling ad blocking. If set to `youtube`, adds a step for enabling YouTube-only ad blocking.