Overhaul web-platform-tests documentation

[gsnedders]

Given we don't have an issue for this and it appears in the Chrome team's Ecosystem Infra team's Q1 OKRs:

From TPAC, seems like browser vendors think the web-platform-tests documentation is way too verbose, while others have no idea where to start.

See also: w3c/csswg-drafts#2438 and all the label:docs issues.

@boazsender can you assign this to whomever is working on it at Bocoup?

[foolip]

@gsnedders, right now we're not really planning anything concrete on this, and I think that maybe a first good step would be to go through all of the open docs issues and figure out what's most important. I don't think an umbrella issue will necessarily help since we already have the label.