Data Managers meeting discussion point #3: url

**Proposed changes to `url`**
The proposed changes introduces a limit to 1 URL linking to the relevant resource per metadata record as per science-on-schema guidelines. If the record would need to represent multiple URLs, these would have to be collected on a single landing page.
Providers are still welcome to provide multiple URLs linking to relevant resources using the field [`distribution`](https://github.com/ESIPFed/science-on-schema.org/blob/master/guides/Dataset.md#distributions) as per science-on-schema.org guidelines.