Add new GraphQL type for milestones

- added the milestone type to be displayed with investments
- the corresponding spec

docs/en/features/graphql.md

@@ -133,6 +133,7 @@ The models are the following:
 | `Budget`                | Participatory budgets         |
 | `Budget::Investment`    | Budget investments            |
 | `Comment`               | Comments on debates, proposals and other comments |
+| `Milestone`             | Proposals, investments and processes milestones |
 | `Geozone`               | Geozones (districts)          |
 | `ProposalNotification`  | Notifications related to proposals |
 | `Tag`                   | Tags on debates and proposals |