docs: add Immutable Tags article (#165)

Fixes #164

Signed-off-by: Michał Sochoń <[email protected]>

mkdocs.yml

@@ -119,6 +119,7 @@ nav:
     - CI/CD Pipeline: articles/building-ci-cd-pipeline.md
     - Authentication and Authorization: articles/authn-authz.md
     - Verifying Image Signatures: articles/verifying-signatures.md
+    - Immutable Tags: articles/immutable-tags.md
     - Software Provenance Workflow Using OCI Artifacts: articles/workflow.md
     - Security Posture: articles/security-posture.md
     - Storage Planning: articles/storage.md