[8.x] [Dashboard/Markdown] Add tabindex (#197848) (#198394)

# Backport

This will backport the following commits from `main` to `8.x`:
- [[Dashboard/Markdown] Add tabindex
(#197848)](#197848)

<!--- Backport version: 9.4.3 -->

### Questions ?
Please refer to the [Backport tool
documentation](https://github.com/sqren/backport)

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BACKPORT-->

Co-authored-by: Krzysztof Kowalczyk <[email protected]>

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>
   );