Consider adding `index.yml` to control item levels.

An `index.yml` might look like:

```yaml
- REQ001:
  - REQ003:
  - REQ007:
- REQ002:
  - REQ004:
    - REQ005:
    - REQ006:
```

One or more of the following could apply:

- `index.yml` replaces `Item.level`
- `index.yml` is read first then applied to each `Item.level`
- `index.yml` is optional