Unify our SLO documentation

We have two documents related to SLOs:

- https://packit.dev/docs/service-level-objectives

- https://packit.dev/research/monitoring/error-budgets/SLOs-SLIs


Let's unify it into one documentat that's official and not just research.

Also make sure to include why we chose those Os and Is for sake of guiding other teams with their SLOs.