Docs:Add gitlab_group_badge usage guide with placeholders

Summary

• GitLab has a badges and placeholders.
  • But terraform provider does not have example usage.
• It would be more useful to show example usage of placeholders in the documentation.

Benefits

• Users will be able to easily start using badges
  • Each projects will be able to use badges automatically.

Example

resource "gitlab_group" "foo" {
  name = "foo-group"
}

resource "gitlab_group_badge" "gitlab_pipeline" {
  group     = gitlab_group.foo.id
  link_url  = "https://gitlab.com/%%{project_path}/-/jobs/artifacts/main/browse"
  image_url = "https://gitlab.com/%%{project_path}/badges/%%{default_branch}/pipeline.svg"
}

resource "gitlab_group_badge" "gitlab_coverage" {
  group     = gitlab_group.foo.id
  link_url  = "https://gitlab.com/%%{project_path}/-/jobs"
  image_url = "https://gitlab.com/%%{project_path}/badges/%%{default_branch}/coverage.svg"
}

resource "gitlab_group_badge" "gitlab_release" {
  group     = gitlab_group.foo.id
  link_url  = "https://gitlab.com/%%{project_path}/-/releases"
  image_url = "https://gitlab.com/%%{project_path}/-/badges/release.svg"
}