Add examples for alternative text in Quarto slides template

[Bai-Li-NOAA]

Thank you for creating the Quarto slides template. It's amazing! It would be very helpful if the template included examples on how to provide alternative text for figures and images to improve accessibility. For example, it would be great to show how to add alt text for an image like light-brand.png in this example:

![`brand: slides-light-brand.yml`](images/light-brand.png){fig-alt="Alternative text."}