StackStorm-HA Docs: Update reference links for Mongo/RabbitMQ dependencies helm charts for accuracy

[arms11]

MongoDB and RabbitMQ dependencies have been updated to use Bitnami based helm charts for the K8s deployment model. However, their links in documentation still reference to old stable repo based references.

MongoDB - https://docs.stackstorm.com/install/k8s_ha.html#id4

• Current: https://github.com/helm/charts/tree/master/stable/mongodb-replicaset
• Expected: https://github.com/bitnami/charts/tree/master/bitnami/mongodb

RabbitMQ - https://docs.stackstorm.com/install/k8s_ha.html#id5

• Current: https://github.com/helm/charts/tree/master/stable/rabbitmq-ha
• Expected: https://github.com/bitnami/charts/tree/master/bitnami/rabbitmq