WebPlatform WPD namespace content

To be used with webplatform/docs.

To review

The following cases has to be fixed.

[] Pages that are in WPD/WPD should redirect with "WPD" only once.
[] Page originally at WPD:Infrastructure/proposals/Site status problem notification messages (id: 12041) is deleted.
[] Page originally at WPD:Browser Testing (id: 8135) is deleted