Add alt text to images in write up

[K-Meech]

Once the report is complete, we should make sure we have alt text for all the included plots.

[dstansby]

Some very basic alt-text was added in c36f24a. I'll leave this open though, because I think it needs expanding a bit to describe what's in the graphs (cc @ruaridhg, if you have time? definitely prioritise the grant report though!). https://a11y.canada.ca/en/alternative-text-and-long-description-best-practices/index.html seems to be a good guide to what to add for figures.