Remove links to grizzly and incubation pages

Grizzly is not maintained anymore, the incubation page is empty.
Therefore do not link to them from our dropdown menus.

Also, remove incubation/index.html completely and redirect the page.

Change-Id: I7db1c5e718620aee16ae7c9f04ea3b37045e0261
Merge "Remove links to grizzly and incubation pages"