feat: add initial drafts for payment and voucher authorization guides

Author: Le-Caignec

src/overview/how-to-pay-for-web3mail.md

@@ -0,0 +1,10 @@
+---
+title: How to Pay for Web3mail
+description: How to pay for Web3mail services
+---
+
+# How to Pay for Web3mail
+
+This page is under development.
+
+<!-- TODO: Add the guide for paying for Web3mail -->

src/use-iapp/voucher-guide.md

@@ -0,0 +1,10 @@
+---
+title: Voucher Authorization Guide
+description: Voucher Authorization Guide
+---
+
+# Voucher Authorization Guide
+
+This page is under development.
+
+<!-- TODO: Add the voucher authorization guide -->