docs: BOLT12 offer and BIP-353 fetching (#711)

docs/SUMMARY.md

@@ -11,6 +11,7 @@
 - [⏩ 0-conf](0-conf.md)
 - [⚠️ Common Mistakes](common-mistakes.md)
 - [🪝 Webhooks](webhooks.md)
+- [⚡ BOLT12](bolt12.md)
 - [🚢 Backend Deployment](deployment.md)
 - [🐳 Regtest Environment](regtest.md)
 - [🔙 Back to Docs Home](https://app.gitbook.com/o/SPM8mTvJyc7OIzGL3HD7/s/XUXMXrD8YTzvuqvAeom7/)

docs/bolt12.md

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+---
+description: >-
+  Information about Swaps with the new BOLT12 standard and some accompanying
+  tips & tricks for how to handle BOLT12 offers
+---
+
+# ⚡ BOLT12
+
+## Swapping to BOLT12 invoices
+
+We allow swaps to BOLT12 invoices in Submarine Swaps. The flow is the exact same
+as with BOLT11 invoices.
+
+### Offers
+
+To make it as easy as possible for clients to fetch BOLT12 invoices for offers,
+we have an easy-to-use
+[API endpoint](https://api.boltz.exchange/swagger#/Lightning/post_lightning__currency__bolt12_fetch).
+
+To verify the returned invoice is actually for the provided offer, the signing
+key of the invoice has to be checked. That singing key has to be either:
+
+- the signing key of the offer, if that is defined, or
+- the public key of the last hop of one of the message paths
+
+Reference implementations:
+
+- [Rust in boltz-client](https://github.com/BoltzExchange/boltz-client/blob/63a2bdd8a7729d45fe0c9b7a016a847bc5a83976/lightning/lib/bolt12/src/lib.rs#L113)
+- [TypeScript in boltz-web-app](https://github.com/BoltzExchange/boltz-web-app/blob/f94e7cdb31946ccbc4bd5d2f4f29086ca63c7335/src/utils/invoice.ts#L261)
+
+## BIP-353
+
+A common way to share BOLT12 offers is via
+[BIP-353](https://github.com/bitcoin/bips/blob/master/bip-0353.mediawiki). After
+[resolving the BIP-353](https://github.com/bitcoin/bips/blob/master/bip-0353.mediawiki#resolution),
+read the URL query parameters. BOLT12 offers are in the `lno` parameter, so if
+it's set, a swap can be created by fetching an invoice and creating a Submarine
+Swap with it.