Sidebar update and overview links

Author: ntombing

docs/topics/merchants/online/sdd/index.mdx

@@ -280,6 +280,7 @@ They can investigate the reason for return and take whatever action is necessary
 - [Accept payments with SEPA Direct Debit (step-by-step)](./guide-sdd.mdx)
 - [Request SEPA Direct Debit payment method](./guide-request.mdx)
 - [Declare a payment mandate](./guide-declare.mdx)
+- [Get SEPA Direct Debit mandate URL](./guide-get-mandate.mdx)
 - [Initiate a SEPA Direct Debit transaction](./guide-initiate.mdx)
 - [Update SEPA Direct Debit payment method](./guide-update.mdx)
 - [Sandbox](./sandbox.mdx)

sidebars.js

@@ -321,6 +321,7 @@ module.exports = {
                 "topics/merchants/online/sdd/guide-sdd",
                 "topics/merchants/online/sdd/guide-request",
                 "topics/merchants/online/sdd/guide-declare",
+                "topics/merchants/online/sdd/get-mandate",
                 "topics/merchants/online/sdd/guide-initiate",
                 "topics/merchants/online/sdd/guide-update",
                 "topics/merchants/online/sdd/guide-create-link",