Update alt texts in Edit _index.md

• Please check this box if this contribution uses AI-generated content (including content generated by GitLab Duo features) as outlined in the GitLab DCO & CLA. As a benefit of being a GitLab Community Contributor, you receive complimentary access to GitLab Duo.

What does this MR do and why?

This MR improves the accessibility of the GitLab documentation by enhancing the alt text for images. The previous alt text was either generic or missing important context. The updated alt text now provides clear and descriptive information about the images, ensuring better accessibility compliance.

Changes introduced:

• Replaced vague alt text with more descriptive explanations.
• Ensured the alt text follows GitLab’s accessibility guidelines.
• Improved clarity for visually impaired users using screen readers.

References

Related issue: GitLab Issue #479161
GitLab documentation contribution guide: Contribute to the GitLab documentation

Before and After Examples:

Previous alt text:

![CF-Provision-and-Deploy-EC2 diagram](../img/cf_ec2_diagram_v13_5.png)