Add CONTRIBUTING.md and related templates
What does this MR do and why?
This MR adds a CONTRIBUTING.md to the repo to help explain the workflow process and how to contribute to this repo. It also adds some default templates for issues and MRs to help contributors get started.
Related to https://gitlab.com/gitlab-org/analytics-section/product-analytics/analytics-stack/-/issues/139+
Edited by Robert Hunt