Docs for how to connect OpenGB to external APIs

[NathanFlurry]

People want to use OpenGB with external APIs

This is basically a primer on how to use fetch + secrets, similar to how we do for emails & s3

Talk about:

• How to make a fetch request
• How to handle the token to the fetch request
• How to let external APIs authenticate with OpenGB using the tokens module (don't go too in depth)
• Permalinks to examples of where we do this already (email + s3)