Create Merge Request triage guideline for Community contribution

Problem

Current Merge Request triage process for Community contribution is unclear and not defined in a single place.

Proposal

Create "Merge Request triage" page similar to Issue Triage and outline high-level process how MRs will be triaged and what is expected. This page should be a single source of truth and have clear SLOs. Additionally we need to explore edge cases like how long a community MR sits w/o response from the community member after a review before moving it to closed or deciding to finish it ourselves(related to this question in Slack).