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

[maxrjones]

Description of the desired feature

#7275 (comment) 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 <gmt-repository>
dvc remote add origin https://dagshub.com/GenericMappingTools/gmt.dvc
dvc remote modify origin --local auth basic
dvc remote modify origin --local user <your-dagshub-user-name>
dvc remote modify origin --local password <your-dagshub-token>

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

[joa-quim]

Thanks. With this I managed to push the two new PS files for the tests that were committed earlier this afternoon.