Improve documentation

[sveneisenschmidt]

• [x] Restructure documentation

• [x] Add developer documentation #26

• [x] Add command documentation #104

• [ ] Add custom documentation for API endpoints, filters, pagination and ordering

• Frontpage (Single page)

  • Quote
  • Usage (yml + curl)
  • Get support
  • License

• Getting Started

  • Requirements
  • Installation
  • Configuration
  • Usage / API
  • Examples

• Customisation

  • Integration with Third-Party-Services
  • Hosting
  • Templates

• Under the hood

  • Commands
  • Events
  • Webhook
  • Development

• Contributing