Add custom instructions for setting up DVC auth in the contributors guide

**Description of the desired feature**

https://github.com/GenericMappingTools/gmt/pull/7275#issuecomment-1448613980 points out that only providing a link to the DAGsHub instructions isn't sufficient for developers to be able to work with DVC. We should instead add customized instructions for the GMT DAGsHub storage:

```
cd
dvc remote add origin https://dagshub.com/GenericMappingTools/gmt.dvc
dvc remote modify origin --local auth basic
dvc remote modify origin --local user
dvc remote modify origin --local password
```

You can get your token from https://dagshub.com/user/settings/tokens.

After following these instructions, the username and password should be stored in your local .dvc/config file (which importantly is not tracked by git).

**Are you willing to help implement and maintain this feature?** Yes/No