Add content for service mesh (#1486)

content/roadmaps/102-devops/content/105-infrastructure-as-code/100-service-mesh/readme.md

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-# Service mesh
+# Service Mesh
+
+A service mesh, like the open source project Istio, is a way to control how different parts of an application share data with one another. Unlike other systems for managing this communication, a service mesh is a dedicated infrastructure layer built right into an app. This visible infrastructure layer can document how well (or not) different parts of an app interact, so it becomes easier to optimize communication and avoid downtime as an app grows.
+
+<ResourceGroupTitle>Free Content</ResourceGroupTitle>
+<BadgeLink colorScheme='yellow' badgeText='Read' href='https://www.redhat.com/en/topics/microservices/what-is-a-service-mesh'>What's a service mesh?</BadgeLink>