Some accessibility fixes for welcome/home page

[peetucket]

Why was this change made? 🤔

Addresses some of the accessibility issues identified in #3171

How was this change tested? 🤨

Localhost and CI

Does your change introduce accessibility violations? 🩺

I hope it reduces them!

[mjgiarlo]

Should we sit on this until we can convert I hope it reduces them! to We ran a set of tools to verify these issues, and now they're gone, and no new ones are introduced ❓

[ndushay]

if you split this PR into the two commits, I'll merge a PR with the first commit (with one tweak I noted in a comment).

[peetucket]

1. split PR into two, second is here: ensure all popovers have aria-describedBy #3186
2. removed <a name="content"> tag based on Mike's comments

[ndushay]

approving this. Agree that it would be good to have a tool to show us this fixed something and didn't break anything else.