Roihu/DL2026: Ideas and needs for new Docs CSC pages

[rkronberg]

With the upcoming infrastructure renewal, we will need to create some new pages to Docs CSC. Examples include documentation for new tools and applications, migration tutorials and so on. Let's use this issue to collect ideas and wishes.

Main PR for Roihu-related updates to Docs CSC: #2597

[rkronberg]

Create one main migration guide, update and improve data/moving/*.md pages (moving data directly between HPC systems, how to deal with SSH keys).