Use `index.yml` to manually reorder item levels.

TODO:

- [x] handle bad data in YAML
- [x] handle unkown IDs
- [x] test outlines with missing rows
- [x] use current starting level when reordering

-----

An `index.yml` might look like:

```yaml
- REQ001:
  - REQ003:
  - REQ007:
- REQ002:
  - REQ004:
    - REQ005:
    - REQ006:
```