docs: add pro API usage

docs/SUMMARY.md

@@ -13,6 +13,7 @@
 - [⚠️ Common Mistakes](common-mistakes.md)
 - [🪝 Webhooks](webhooks.md)
 - [⚡ BOLT12](bolt12.md)
+- [🏅 Pro](pro.md)
 - [🤝 Referral Program](referral-program.md)
 - [🚢 Backend Deployment](deployment.md)
 - [🐳 Regtest Environment](regtest.md)

docs/pro.md

@@ -0,0 +1,27 @@
+---
+description: >-
+  Boltz Pro is the same as our main site, but with the crucial difference that
+  fees are variable and set based on our liquidity needs
+---
+
+# 🏅 Pro
+
+Boltz Pro is an experimental service designed to dynamically adjust swap fees
+based on our liquidity needs, helping maintain balance between our wallets and
+Lightning channels. We temporarily offer incentives (like 0% or even negative
+fees) to encourage users to conduct swaps that work in favor of our liquidity
+needs.
+
+## Web Usage
+
+A web app for Boltz Pro is available on
+[pro.boltz.exchange](https://pro.boltz.exchange/).
+
+### API Usage
+
+The Boltz Pro API uses the
+[exact same endpoints and methods as the normal one](api-v2.md). Only two things
+need to be done to use Pro:
+
+- add a `Referral` header to all pair related `GET` requests and set it to `pro`
+- set the `referralId` to `pro` when creating swaps