Migrate to new documentation

As of v2.3.0 the documentation was migrated into the main repository, using docusaurus instead of vuepress.

Tasks

• check documentation for parts that need to be adjusted
  • Introduction / About us
  • User Manual
    • "our" online demo
    • How to get access
    • Register organisation
  • Any hard links to https://research-software-directory.org
• if required migrate content from https://codebase.helmholtz.cloud/research-software-directory/rsd-documentation to new documentation
• archive https://codebase.helmholtz.cloud/research-software-directory/rsd-documentation
• provide new documentation image