Docker: MongoDB: Add MongoDB Replica Set

[dbazhenov]

A Docker Compose configuration for MongoDB 8 Replica Set with three replicas was tested.

https://gist.github.com/dbazhenov/fd47167734230d294a4aa10da623d1f2

Add it to the examples in the docker folder and Readme.

Blog Post
https://percona.community/blog/2025/03/18/setting-up-and-monitoring-mongodb-8-replica-sets-with-pmm-3-using-docker-a-beginner-friendly-guide/