docs: improve Build Service README with architecture and usage details

[nihar-landge]

Summary

The existing README was missing key information and had some inaccuracies that could confuse new contributors and operators. This PR improves clarity, accuracy, and completeness.

Changes made

• Added Table of Contents and section dividers for easier navigation
• Split installation instructions into Production vs Development/Standalone sections
• Added Verifying the Installation section with accurate kubectl commands using kubebuilder-standard deployment selectors
• Converted operating modes and PipelineRun parameters to tables for readability
• Clarified default controller behavior when no annotation is set on a Component CR
• Softened Renovate dependency wording to be platform-neutral (platform-dependent rather than claiming it must be separately installed)
• Removed Renovate from the Dependencies table to avoid inaccurate framing
• Removed hardcoded pipeline count ("five pipelines") to prevent docs going stale across releases
• Added make run for local testing without deploying to cluster
• Expanded Contributing section with dev commands and code of conduct link

Testing

Documentation-only change. No code was modified.

Fixes #544

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[nihar-landge]

Hey @MartinBasti @StanislavJochman 👋

I've opened this PR to improve the Build Service README with better
structure, accurate verification commands, and clearer architecture
details. Would appreciate a review when you get a chance!