docs(backend): add backend component overview section

[RATNAPRADEEP]

Description

Adds a Backend Component Overview section to the backend README to improve contributor onboarding and provide a clearer understanding of backend architecture and component responsibilities.

Checklist

• Signed off my commit (DCO)
• Follows PR title convention
• Documentation only change

[google-oss-prow]

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[github-actions]

🎉 Welcome to the Kubeflow Pipelines repo! 🎉

Thanks for opening your first PR! We're excited to have you onboard 🚀

Next steps:

• Check out the Contributing Guide for repo-specific guidelines and the Kubeflow Contributor Guide for general community standards
• Join our #kubeflow-pipelines Slack channel
• Attend the Kubeflow Pipelines WG meetings

Feel free to ask questions in the comments.

[google-oss-prow]

Hi @RATNAPRADEEP. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[Copilot]

Pull request overview

This PR adds a "Backend Component Overview" section to the backend README to improve contributor onboarding by documenting the key backend components and their responsibilities in the Kubeflow Pipelines architecture.

Changes:

• Added a new "Backend Component Overview" section that lists and describes six major backend components (API Server, Driver, Launcher, Persistence Agent, Metadata Writer, and Cache Layer)
• Included explanatory text about how understanding these components helps contributors

[Copilot]

The Driver description as "Translates pipeline specifications into executable workflow definitions" may be imprecise. Based on the code in backend/src/v2/driver/driver.go and backend/src/v2/cmd/driver/main.go, the driver appears to be responsible for orchestrating task execution (ROOT_DAG, DAG, CONTAINER types) and managing execution state with MLMD, rather than translating specifications.

The pipeline-to-workflow translation is typically done by the compiler component. Consider revising this description to something more accurate like "Orchestrates task execution and manages execution state" or "Prepares and manages the execution context for pipeline tasks".

Suggested change

	- **Driver** – Translates pipeline specifications into executable
	workflow definitions.
	- **Driver** – Orchestrates task execution and manages execution state
	with ML Metadata (MLMD).

[hbelmiro]

@RATNAPRADEEP can you please address copilot's comments?

[RATNAPRADEEP]

Addressed Copilot comments. Added Scheduled Workflow Controller and Viewer Controller to the Backend Component Overview. Please review.