Need to document W3C entries obsoleted by WHATWG specs

[tidoust]

The tr.rdf file (same for the W3C API) only knows about W3C specs. As such, it does not contain the information needed to fill the obsoletedBy properties of W3C specs that got superseded by WHATWG specs. When the shortname stayed the same, the data essentially moved to whatwg.json, which seems fine, but when the shortname changed in the process, the relationship gets lost in data. Main missing links:

• cors -> fetch
• html5 entries -> html
• xmlhttprequest entries -> xhr

[JohnRiv]

Another example that will need to be handled: 2dcontext -> html

I'm not sure when https://www.w3.org/TR/2dcontext/ started redirecting to HTML, but I just noticed it does now (and it has its own section in it: https://html.spec.whatwg.org/multipage/canvas.html#2dcontext)