[Guides] AT Protocol (Bluesky) integration guide

[jstanden]

Depends on: jstanden/cerb#1806

Once implemented, we need a guide for integrating with AT Protocol apps using connected services/accounts. This starts with an app-specific password that is exchanged for JWT tokens for subsequent requests.

See:

• https://docs.bsky.app/docs/get-started#create-a-session
• https://docs.bsky.app/docs/advanced-guides/posts
• https://lifehacker.com/tech/why-you-should-be-using-bluesky-app-passwords