Mirror definition - Proposal for new mirror administration routines and mirror requirements

Before this proposal, all tasks surrounding mirrors were manual and in free-text form. This RFC therefore outlines the definitions and guidelines surrounding both becoming but also maintaining and decommissioning a mirrors in a new format using TOML and GitLab.

This RFC is an effort from @pitastrudl, @wahrwolf, @torxed, @dvzrv and the community.