Fix class hierarchy diagram and docs for explicit nulls

[bishabosha]

http://dotty.epfl.ch/docs/reference/other-new-features/explicit-nulls.html

currently the link for the class hierarchy image is broken, and there are no links in the sidebar

[odersky]

Actually, the type hierarchy image still does not show.

[odersky]

In fact the diagram is outdated, and the surrounding text as well. Null is now a value type, not a subtype of Any. The docs and the diagram need to be fixed.

[odersky]

The link is good now, but the text and diagram need to be fixed.

[noti0na1]

@odersky Currently, Null is still a subtype of Any.

I tried to simply change the parent of NullClass to AnyValType, but this doesn't work. There are some errors in the Erasure phase.

Should we also add it to ScalaValueTypes?