[EPIC] Group-level wiki
## Problem
When working with multiple projects managed via GitLab groups, there is the potential to add documentation that spans multiple projects in a wiki. Right now you can't do that.
Places to add Wiki:
* Project-level wiki documents and holds information related to the project (exists)
* Group-level wiki documents and holds shared information related to a group, containing projects (and sub-groups) - This proposal
* Subgroup-level should also be addressed in this proposal (@dsatcher @toupeira did we think about this?)
* Global wiki's (not a part of this) will be doable after MVC 2 (see issue https://gitlab.com/gitlab-org/gitlab/issues/15017)
## Proposal
**This epic outlines MVCs on the way to full group-level and global-level wiki support.**
## Open Questions
* When to call the [main Group-level issue](https://gitlab.com/gitlab-org/gitlab/issues/13195) as **done** (600+ upvotes)?
* After one wiki group support (MVC 1)?
* After multiple wiki's (MVC 2)?
* After a allowing the customer to import or migrate from Project to Group? (MVC 3)
* Do we move the HAML > vue.js convert (nested at the top of MVC 2) to the editing epic or keep it inline here and get it up to speed? @rkuba can we move this out out? The rest of the MVC has HAML tasks so I may have your issue in the wrong place
## What we're NOT doing
* GraphQL or modernizing the underlying structure
* This will support other endeavors around modernizing the editing experience but that's not happening here.
<details>
<summary>Older notes</summary>
#### Wiki only projects
- Better empty state https://gitlab.com/gitlab-org/gitlab-ce/issues/48731
- Improve creation workflow possibly using a group template?
#### Access from group
- Pinned projects https://gitlab.com/gitlab-org/gitlab-ce/issues/28816
#### Interested Customers
- https://na34.salesforce.com/00161000006fkPe; will help them migrate off of external tools where multi-project documentation is currently stored.
</details>
epic