Add guidelines for loading patterns 🔄

[maximedegreve]

trafficstars

Problem There are currently no guidelines on how to use loading states throughout GitHub.

Considerations

• Toasts that load forever
• Forms that load for a long time
• Should buttons be allowed to have a spinner inside them
• How and when do we show a timeout
• Possible new skeleton loaders for fast loading content and reducing layout shifts
• How do we do all above in an accessible way.

Original Slack thread (Internal)

[howdyray]

Adding some explorations that @cfahlbusch did for loading on the mobile apps.