Replace person with team for documentation reviews (#3420)

* Replace person with team for docs reviews; see (internal) https://github.com/grafana/technical-documentation/issues/520.

1. Remove `Docs Squad`.
2. Create team `grafana/docs-metrics`.
3. Add team members; right now it is just the default (me).
4. Add `grafana/docs-metrics` to the list of collaborators.
5. Select `Write` permissions.

CODEOWNERS

@@ -2,4 +2,4 @@
 * @grafana/mimir-maintainers
 
 # Documentation.
-/docs/ @osg-grafana @grafana/mimir-maintainers
+/docs/ @grafana/docs-metrics @grafana/mimir-maintainers