Alt text is not descriptive [gen2 pages]

**Describe the content issue**:
Images listed below do not have descriptive/informative alt text. For markdown the alt text is located between the square brackets, ``.

The list below isn't exhaustive since I found these images based on the word count for the alt text, but it should be a good starting point to make our alt text better for people who use screen readers.

Some resources to help write alt text:
- https://webaim.org/techniques/alttext/
- https://www.a11yproject.com/posts/are-you-making-these-five-mistakes-when-writing-alt-text/

https://github.com/aws-amplify/docs/blob/main/src/pages/gen2/deploy-and-host/fullstack-branching/monorepos/index.mdx:
```
52
53: 
54
```

https://github.com/aws-amplify/docs/blob/main/src/pages/gen2/deploy-and-host/sandbox-environments/setup/index.mdx:
```
21
22: 
23
```

https://github.com/aws-amplify/docs/blob/main/src/pages/gen2/how-amplify-works/concepts/index.mdx:
```
130
131: 
132
```

https://github.com/aws-amplify/docs/blob/main/src/pages/gen2/start/mobile-support/index.mdx:
```
872
873: 
874
```

Start by reviewing the four listed .mdx files and the referenced images: monorepos/index.mdx, sandbox-environments/setup/index.mdx, concepts/index.mdx, and mobile-support/index.mdx. Check each image's content, replace the listed alt text with descriptive text for screen-reader users, and verify that all four affected images have informative alt text.

Written by the indexing model from the issue text.