docs(v1): add comprehensive piped configuration reference

[shivansh-gohem]

What this PR does / Why we need it:
This PR provides the structured and updated v1 configuration reference for the Piped agent (piped.config).

Note: This PR was split from #6584 per maintainer request (@eeshaanSA) to keep the Application and Piped configuration reviews properly isolated.

Key Documentation Updates:

• Documented the shift to the new v1 plugin-based architecture.
• Replaced legacy platformProviders with the new plugins and nested deployTargets specifications.
• Added detailed, structured markdown tables for internal configurations like SecretManagement, EventWatcher, and Git.
• Added comprehensive routing tables for NotificationRoute and NotificationReceiver (including NotificationReceiverSlack and NotificationReceiverWebhook per review feedback).
• Cleaned out outdated v0 configuration references.

Which issue(s) this PR fixes:
Split from #6584
Fixes #6542