Update alt text for images

This issue is to improve the alt text for images in the documentation to improve accessibility. This issue is similar to issue #479161 in GitLab.org.

Alt text examples

Examples of helpful alt text:

• ![Setting up merge request approvers](/images/7_12/approvals_settings.png)
• ![Using merge request approvers](/images/7_12/approvals_mr.png)

Examples of alt text that can be improved:

• ![alt text](./Deployment_Options_GitLab_Dedicated.png "GitLab Deployment Options")
• ![alt text](shift_left.png "Shift Left Model")