Review signup (account/socialacc) flow

[jh-RLI]

Description of the issue

The signup workflow should be adapted to best practices. Focus on sociallogin if the user is missing some important information like e-mail. See codebergs "Complete Registration".

Ideas of solution

• Document the current flow
• check how others do it
  (see GWDG / academic cloud / CodeBerg)

Workflow checklist

• I am aware of the workflow in CONTRIBUTING.md