Add TU Delft research support staff onboarding guide

[aysunrhn]

This PR adds a TU Delft wide research support staff onboarding guide to the DCC Guides with a shared competence framework and advanced training resources.

The content should be applicable to all research support roles (RSEs, RDEs, data stewards, research IT, library support, Open Science, project/grant support), while still fitting naturally within the DCC Guides as a central reference hub.

Major changes:

• index.md: new Research Support Onboarding landing page and supporting subpages
• about.qmd: research support ecosystem at TU Delft + how RSE practices fit
• contacts.qmd: support landscape + routing (“where to ask what”)
• learning.qmd: expanded learning section (see below)
• soft_skills.qmd: narrative structure + callouts + practical language patterns

The ultimate goal with this section

• The DCC Guides are a natural entry point for shared practices, but we lacked an onboarding narrative and competence framing that works beyond the DCC team.
• This does not attempt to formalize TU Delft policy or HR requirements.
• It provides a practical onboarding reference and a shared learning vocabulary that teams can adapt locally.

TODO

• Add/replace contact points if there is any missing
• Add cross-links to existing DCC Guides pages where appropriate
• Expand the learning section with a curriculum or a possible career trajectory for research support staff