Alt text is not descriptive [how amplify works page]

**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/[platform]/how-amplify-works/index.mdx:
```
182
183: 
184

198
199: 
200
```