fix: add missing index link for DNS

Author: ntse

source/index.html.md.erb

@@ -46,6 +46,7 @@ This is the technical documentation for the developers and DevOps engineers work
 - [AWS Account Structure](documentation/aws/account-structure)
 - [AWS Centralised Backups](documentation/aws/centralised-backups)
 - [AWS Networking](documentation/aws/networking)
+- [AWS DNS](documentation/aws/dns)
 - [Delivery and Operations](documentation/aws/delivery-and-operations)
 
 ## Other documentation