Allow Collections To Be Nested / Grouped

**Is your feature request related to a problem? Please describe.**
Currently all collections are displayed at the top level of the menu in the sidebar. This means that a collection like Pages is at the same level of hierachy as StaffMembers which might only be used on a single page.

**Describe the solution you'd like**
I'd like control over nesting of Collections, so a page-specific Collection or Collections could be nested inside a group for that Page

**Describe alternatives you've considered**
There is no available alternative.

**Additional context**

I'd like the ability to have the sidebar look something like:

```
- Pages
- About Page
- Staff List
- About Slides
- Contact Page
- Contact Details
```