Description
Document credential systems. Currently scattered across multiple docs in utorials and partly getting getting. Need a new concept doc plus rework of existing tutorials and potentially changing the type to how-to.

Tasks:

• content/docs/concepts/credential-system-concept.md (NEW concept) - Credential resolution, consumer types, config structure
• content/docs/tutorials/creds-in-ocmconfig.md (REVIEW tutorial, potentially change to how-to) - Align with credential-system concept
• content/docs/tutorials/configure-credentials-for-controllers.md (REVIEW tutorial) - Consistency check with credential-system

Done Criteria

• Credential concept doc
• Existing credential tutorials aligned with new concepts
• No duplicate content
• All relrefs updated