Audit website for accessibility issues

[JoshuaKGoldberg]

It's been a while since the website was designed or significantly updated. Web accessibility practices and detection tools have changed a lot over the last decade! I ran aXe via Lighthouse on a couple of pages:

• yeoman.io: 23/25 checks passed
• https://yeoman.io/learning: 24/25 checks passed

Regardless of aXe passing checks, we'd also want to run through the site with a screenreader to make sure it's usable in that way.

Filing this issue to track that someone should:

1. Run aXe (or an equivalent) on the site's pages and file issues for each classification of accessibility issue reported
2. Navigate through the site's pages with a screenreader and file issues for each classification of accessibility issue they discovered

I.e. this is a meta-issue: I'm suggesting smaller, granular issues be filed as a result of it. Once both (1) and (2) are confirmed as being done I think we can close this issue out.