pull: add troubleshooting hints for failed command

[bobertlo]

It has been mentioned (treeverse/dvc#4316) that the error text when checkout fails after pulling from an out of date remote is not very clear. In attempting to clarify the error (treeverse/dvc#4339) it was suggested that a link to a "common issues" page would be helpful.

Looking at the website, I agree that https://dvc.org/doc/user-guide/troubleshooting might be a good place to put this.

[jorgeorpinel]

Sure @bobertlo ! For now in the core repo please use a link in the form https://error.dvc.org/short-problem-desc. This will redirect to https://dvc.org/doc/user-guide/troubleshooting#short-problem-desc

Then please let us know here what the value of <short-problem-desc> ends up being, and a description of the problem/solution (or link to where the solution is described), so this issue is actionable.

Or, feel free to add the new section in content/docs/user-guide/troubleshooting.md — notice the H2 header in Markdown should be in the form ## Problem title {#short-problem-desc}

Thanks