user-guide: add "Define remote through another remote"

[efiop]

From treeverse/dvc#1614
Related #108

[jorgeorpinel]

Where do you think this example, note (or full page?) could be located @efiop? Other than in get/import (that's #108). Thanks

[efiop]

@jorgeorpinel Again, sorry for the delay. I think it might be worth creating a user-guide page for it and reference it in all the commands that support remote:// notation.

[jorgeorpinel]

No worries, thanks for the follow up. I read treeverse/dvc#1614 (comment) and I think probably this would be a relatively short note or section so not sure its worth a full new guide. Maybe just another section of dvc remote add

[shcheklein]

This is too complicated, I haven't seen a need for this tbh. Let's not document this for now.

[efiop]

Well, users are using this and this is the only appropriate method available right now for dvc import-url. Reopening.