[Dashboard/Markdown] Add tabindex (elastic#197848)

## Summary

This PR adds `tabIndex` to
[markdown_vis_controller.tsx](https://github.com/elastic/kibana/compare/main...kowalczyk-krzysztof:kibana:fix/dashboard-markdown-panel?expand=1#diff-47267cf2f7d8f9e72e157ddb40292226a88ad5e51f9c92486799d381c5085e5f).

Closes: elastic#186559

## Visuals:


https://github.com/user-attachments/assets/8264c584-e7ca-4647-946a-7b9bd9f7aa8f

This video demonstrates the markdown panel being tabbable and the
scrollbar in being controlled with keyboard.

Author: kowalczyk-krzysztof

src/plugins/vis_type_markdown/public/markdown_vis_controller.tsx

@@ -31,6 +31,7 @@ const MarkdownVisComponent = ({
         data-test-subj="markdownBody"
         openLinksInNewTab={openLinksInNewTab}
         markdown={markdown}
+        tabIndex={0}
       />
     </div>
   );