Feature flag rollout docs: major rewrite
This started as some minor updates in 1/2:
The template said "Remove the...", but this is for creating features. We've also changed the slack channel name. Finally, link to some example dashboards.
But I kept going, and in 2/2 much of the docs are rewritten so as to cross-link the checklist in the issue template with more detailed descriptions in the docs.
Edited by Ævar Arnfjörð Bjarmason