Docs/cicd improve

[radheshpai87]

PR: docs(ci): add CI/CD docs and README pointer (Fixes-371)

Fixes-371: https://github.com/docker-slim/docker-slim/issues/371

What

• Add docs/CI/CD.md — expanded, provider-specific CI/CD guidance (GitHub Actions, GitLab CI, Jenkins, dind notes, troubleshooting, flags).
• Add docs/ci/cd.md — compact, lowercase-path copy intended for quick edits.
• Update README.md to point to docs/CI/CD.md and remove the large duplicated CI/CD section.

Why

• Centralize CI/CD guidance in a dedicated docs file for easier maintenance.
• Provide a small, easy-to-edit docs/ci/cd.md so contributors can quickly make fixes.
• Reduce duplication in README.md and keep the README concise.

How Tested

• Verified the new files were created on branch docs/cicd-improve and committed:
  • docs/CI/CD.md (detailed guide)
  • docs/ci/cd.md (compact, editable copy)
  • README.md (updated pointer)
• Quick manual review for formatting and example correctness.