How to build Ghost Integrations?

Integrations aren't like plugins. Ghost is very flexible, so most tools can be integrated with it using any of the following methods:

• Paste a link for an embed (OEmbed)
• HTML card embeds
• Zapier integration
• Code injection
• Integrate directly into a theme
• Custom integrations using webhooks: https://ghost.org/integrations/custom-integrations/
• Advanced integrations using the Ghost API https://ghost.org/docs/content-api/
  https://ghost.org/docs/admin-api/

The integrations library is a curated list of the most common integrations that Ghost users make use of. They're a series of guides about how to integrate tools with Ghost using the methods above.

What sort of integration?