Feature request: global links

I like using reference style markdown links e.g.

```
[ref_name]: https://my-link-to-thing
```

It would be nice if I could, whether in `index.ipynb` or `ini` define a list of global reference links that I can use across all other notebooks without having to re define them everywhere.

Start by checking how nbdev reads index.ipynb and ini configuration, then trace how reference-style Markdown links are handled across notebooks. Define what a global reference-link source should be and verify that links declared there resolve in other notebooks without repeated definitions.

Scritto dal modello di indicizzazione a partire dal testo della issue.