docs: Document Storybook a11y addon usage (#1116)

The Storybook Accessibility Addon was already installed and configured in the project. This change adds the missing documentation to `STORYBOOK.md` explaining how to use the addon to identify accessibility issues.

A pre-existing issue causes the `build-storybook` command to fail, but this is unrelated to the a11y addon configuration.

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>

Author: google-labs-jules[bot]

STORYBOOK.md

@@ -28,6 +28,21 @@ npm run storybook
 npm run build-storybook
 ```
 
+## Accessibility Testing
+
+This project uses the Storybook Accessibility Addon (`@storybook/addon-a11y`) to help identify and fix accessibility issues in components during development.
+
+### How to Use the Addon
+
+1.  **Open a Story**: Navigate to any component story in the Storybook UI.
+2.  **Select the Accessibility Tab**: In the addons panel at the bottom of the screen, click on the "Accessibility" tab.
+3.  **Review the Results**: The panel will display a list of automated accessibility checks. It highlights:
+    *   **Violations**: Issues that fail accessibility standards (e.g., WCAG). These should be fixed.
+    *   **Passes**: Rules that the component correctly adheres to.
+    *   **Incomplete**: Rules that could not be automatically checked and may require manual verification.
+
+By using this tool, you can catch common problems like incorrect color contrast, missing ARIA attributes, and improper element structures, ensuring our components are usable by as many people as possible.
+
 ## Benefits
 
 - **Isolated Development**: Test components without full app context