Support for: [LINK TEXT](ref:<KEY>)

It's for content that's reused and has different link destinations depending on the page's location.

Currently, the transformer does this:
```
[alert rule](ref:create-gm-alert-rule)
```

Should do:
```
[alert rule](http://grafana.com/docs/grafana/next/alerting/fundamentals/alert-rule-evaluation/)
```