Page to act as a group

[jdkandersson]

The following navigation table should be supported:

| 2 | h-upgrade | [Upgrade](/t/upgrade-intro) |
| 3 | h-major-upgrade | [Major upgrade](/t/11750) |
| 3 | h-major-rollback | [Major rollback](/t/11751) |

Currently this won't work because a group (an entry without a link to a topic) is required for a level change as this is mapped to a directory.

An implementation idea is to have a file of index.md within a group be considered a page that also acts as a group on discourse. This does mean that a topic that would otherwise have been intended to be called index.md would then result in a group topic on discourse.

[jdkandersson]

There isn't a clear mapping between the discourse documentation and what this would look like on git