WIP: Roihu migration guide

[rkronberg]

Proposed changes

New page: Roihu Migration guide

Idea is to collect best practices and recommendations for moving data from Puhti/Mahti to Roihu. We have a lot of good guides already, especially at https://docs.csc.fi/data/moving/ so idea would be to heavily link to these (and ensure the old pages are still up to date e.g. with respect to SSH keys and SSH agent forwarding etc.).

Based on discussion with @jlento it would make sense to structure the guide such that the easiest solutions are given first, these are possibly sufficient for small datasets, and then gradually more complex approaches based on data size (volume and number of files).

https://csc-guide-preview.2.rahtiapp.fi/origin/roihu-migration-guide/

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