Add group-level wiki
## Problem
While each git repository has it's own git powered wiki, but there is cases where it would make more sense to have wiki contained in the group that covers several projects. An example link of how this could be accessed done is as follows...
https://gitlab.com/gitlab-org/wikis/home
## Work and epics
- MVC 1 - Group-level wiki - Support one wiki at group level: https://gitlab.com/groups/gitlab-org/-/epics/2214
- MVC 2 - Feature parity with project wikis (git, search) - https://gitlab.com/groups/gitlab-org/-/epics/2782
### Proposal from @jareko
* Prototype with artboards and specs: https://sketch.cloud/s/z3oja/a/mxl9jk/play
* [Designs for review and discussion](https://gitlab.com/gitlab-org/gitlab/issues/13195/designs)
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