Update credential permissions section to make it more user friendly

[tapadipti]

          Not a blocker, but we should follow up to make this more user-friendly. We can include the credentials video from @dacbd and mention like he does that an easy way to get started is to give full access to ec2 and s3. We could also provide an example policy that grants all the permissions below so people can copy and paste it.

I'm less familiar with GCP/Azure, but we should be able to do something similar.

Originally posted by @dberenbaum in https://github.com/iterative/dvc.org/pull/4589#discussion_r1228271951