Allow Setting readme_url for Groups via API, Supporting External Git Repository References

Description

With initial implementation GitLab Groups API does not support setting or updating a readme_url attribute for a group, nor does it allow referencing a README file hosted in a different Git repository. This limitation makes it difficult for organizations to centralize documentation or provide group-level onboarding and guidelines that are maintained outside the group’s own repositories.

Proposal

• Extend the Groups API to support readme_url attribute to allow this URL to point to any valid external Git repository or raw README file (e.g., a README.md in a documentation repo).
• Provide validation to ensure the URL is accessible and points to a valid markdown file.
• Provide possibility to change readme

Use Cases

• Centralizing onboarding documentation for all projects within a group.
• Reducing duplication of documentation across multiple projects.