Add troubleshooting guide for lfs uploading timeouts
What does this MR do?
This MR revises documentation about lfs uploading.
- Troubleshooting for
Google::Apis::TransmissionError: execution expired
- Add GCS as S3 is not the only available object storage
What are the relevant issue numbers?
Related: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/19581#note_80953219
Edited by Kamil Trzciński