Make the navigation wrapping ul component user customisable

[mikehazell]

The NavigationContainer component rendered it's children inside a ul meaning if you wanted to render anything other than an li you would have to do some gymnastics to make it work.

This change makes it the users' responsibility to properly wrap list items in a ul.

• exposes a NavItemGroup component with appropriate styling
• updates "Custom Admin UI Navigation" usage docs

[codesandbox-ci[bot]]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 20fe773d21d864bf9ed93db583f5f2ade6d65513:

Sandbox	Source
@keystone-6/sandbox	Configuration

[dcousens]

Likely superseded by https://github.com/keystonejs/keystone/pull/9253

[dcousens]

Superseded by #9253, and if not, we'll re-open