Add a guide for alt text

[dmundra]

Are guide is lacking some directions for alt text, we could write a few paragraphs and then link to resources:

• https://www.tpgi.com/what-is-alt-text-and-how-to-write-it/
• https://design.va.gov/content-style-guide/alternative-text-for-images
• https://webaim.org/techniques/alttext/
• https://www.w3.org/WAI/tutorials/images/decision-tree/