Improve accessibility and usability of launch page

[stevepiercy]

I strongly dislike both "default" for "Volto" and "classic" for "Classic UI". I see no benefit to rename things. I see harm in that we have to explain what "default" and "classic" actually mean. This is especially confusing to Classic UI users because to them Classic UI is the default frontend, and Volto is optional.

Below is a rough mockup of what I suggest, but without all the horizontal rules that result from the GitHub Markdown render of my comment and the existing UI.

Finally, the hover over each panel is bad for a11y. We should always show the Create buttons and not replace the image with text.

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Create a new Plone site

Select an available distribution for your site's frontend

Volto                          | Classic UI
Distribution: volto            | Distribution: classic-ui
[Volto screenshot]             | [Classic UI screenshot]
[Create Plone site with Volto] | [Create Plone site with Classic UI]

I will submit a PR in a few days. @davisagli will work on a PR for plone.volto. I will also follow @davisagli's example for plone.classicui.

See discussion: plone/documentation#1733 (comment)

[djay]

Totally agree