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• Cleaned out outdated v0 configuration references.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

[eeshaanSA]

@mohammedfirdouss, @rahulshendre could you please kindly review this PR?

Also, please let us hold this, and wait for #6584 to get merged before we can actually merge this. Thanks!

[eeshaanSA]

@shivansh-gohem, thank you again for this. Let us work towards getting this merged.

PTAL this comment

[mohammedfirdouss]

groups and ignoreGroups are missing from the NotificationRoute table but both fields are still in the v1 struct: https://github.com/pipe-cd/pipecd/blob/master/pkg/configv1/piped.go#L302-L303. They should be added back.

[mohammedfirdouss]

The SecretManagement section says config: object with no breakdown. The v1 config has two types, each with required fields. They still exist

KEY_PAIR: https://github.com/pipe-cd/pipecd/blob/master/pkg/configv1/piped.go#L507
GCP_KMS: https://github.com/pipe-cd/pipecd/blob/master/pkg/configv1/piped.go#L563

[mohammedfirdouss]

port is marked as required? But Validate() only checks name and url
https://github.com/pipe-cd/pipecd/blob/master/pkg/configv1/piped.go#L671

Should be optional.

[mohammedfirdouss]

Removing chartRepositories is correct for v1, but can you add a note saying where they moved?
In v1 they go under plugins[].config for the relevant plugin. @shivansh-gohem

[rahulshendre]

+1 to all of @mohammedfirdouss points.

One addition alongside the chartRepositories migration note: chartRegistries also moved to spec.plugins[].config on the Kubernetes plugin but isn't mentioned in the doc. source

[shivansh-gohem]

Thanks for the thorough reviews, @mohammedfirdouss and @rahulshendre! Great catches on all of these.

I have updated the PR with the following changes:

(i) Added groups and ignoreGroups back to NotificationRoute.

(ii) Added the SecretManagementKeyPair and SecretManagementGCPKMS breakdown tables.

(iii) Changed port in PipedPlugin to Optional.

(iv) Added an explicit note in the intro explaining that chartRepositories and chartRegistries have moved to spec.plugins[].config.

The latest commit is pushed. PTAL!