Description of OIDC based access to dCache

Take over description for access to dcache via rclone / oidc-agent and DESY keycloak.

• 👉 Review here
  • View creds
• 👉 Edit here

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To-do

• Take over rclone / oidc agent specific stuff
• Fix obvious formatting things, link to the menu structure
• Check that the commands do work as described
  • also for rclone mount?
• Brief review understandability of text

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Maybe in a separate branch, to not block this one

• Improve toplevel description/overview, to have the most important user specific infos at once glance