Add `foaf:PersonalProfileDocument`

Some guys at Solid are trying to get WebID to become an official recommendation, by creating a superspec that would encompass the old one and add a reference to JSON-LD, without bringing in breaking changes since it's overused:

https://github.com/solid/solid-wg-charter/issues/39

For the moment, it looks like this:

https://jacoscaz.com/WebID/superspec/webid.html

This sentence concerns us:

> For WebIDs without fragment identifiers, an HTTP request on the WebID *MUST* return the status code `303 See Other` with a `Location` header URI referring to the Identity Document.

So if I understand correctly, to be in the clear, we'd have to create a resource of type `foaf:PersonalProfileDocument` for each WebID, and link it to the WebID via this header 303.

The issue names no project files, tests, or entry points. Start by locating how WebIDs are served and how HTTP 303 responses and Location headers are handled. Then determine how to create and link a foaf:PersonalProfileDocument for each WebID, and verify that requests without fragment identifiers return the identity-document URI.

Written by the indexing model from the issue text.