Microsites Documentation - get started

[finnlewis]

• [x] Start documentation in current docs site by adding a top level menu item 'Microsites'
• [x] Overall features
• [x] Roles
• [x] Adding users / members
• [ ] Enabling / disabling content
• [ ] Site settings
• [x] Adding custom theme

[msayoung]

Made a start here: https://github.com/localgovdrupal/docs/pull/144