Improve default ALT attribute value for images in docs

[ravigadhiya007]

1. Update default 'alt text' to more contextually relevant image descriptions in Gutenberg
2. Enhance image accessibility by updating ALT attribute default in Gutenberg

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Co-authored-by: ravigadhiya007 <[email protected]>
Co-authored-by: Mamaduka <[email protected]>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @ravigadhiya007! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[Mamaduka]

Thanks, @ravigadhiya007!