Run Lighthouse tool and understand the accessibility issues that it flags

[troyraen]

This Lighthouse tool can scan webpages and flag accessibility issues. Run it on the rendered html pages for each of the notebooks listed in #184 and try to fix any issues it flags. It will also suggest items to check manually. Do that as much as possible and try to fix any issues that surface.

[troyraen]

To clarify, the goal is not to spend a lot of time trying to track down every problem/solution right now. A better scope for this issue is to run the scan (including the manual stuff to the extent possible) and sort the items it flags into rough categories, like:

• easy thing we should probably just do
• unclear or hard thing
• thing we don't think we should spend any more time thinking about

Then if there are easy things we can do now, sure let's do them. But just creating a plan or questions that we'd like the consultant's input on would be great. (See parent issue)