[Nim 2.0 Cleanup] hide Not nil docs from the experimental manual

[ringabout]

ref https://github.com/nim-lang/RFCs/issues/437

"Not nil annotation" -- Should be planned for Nim 2.x but removed from the "experimental" manual.

[konsumlamm]

What exactly is the point of this? Now people can't read about notnil at all. I think this should only be removed from the experimental docs when it's actually stable (and then be moved to the manual).

[Araq]

@ringabout I've updated the roadmap.

[ringabout]

I see