Update toc sidebar tutorials copy and image

[veerapaananen]

Update image and copy for Docs sidebar Tutorials -section

[github-actions]

AI Language Review

The main difference between the two versions is in the TocFooter class's render method:

• The heading text in the .new version changes from "Video tutorial" to "UI Training & Certification".
• The descriptive text also changes from "Learn Vaadin fundamentals through short video tutorials." to "Master your Java web app development skills and become a certified Vaadin developer."

Issues to Address:

1. Ensure Consistency: The heading and the description should consistently represent the linked resource. Confirm that the link provided indeed leads to UI training and certification content, as the text implies.
2. Accessibility: Ensure that the alt text for the image ("Learn by watching") is updated appropriately to reflect the changed context (e.g., "Certification Training Material" if relevant). This will help users who rely on screen readers.

No code logic errors are found in the changes. The rest of the file is unchanged.