Add DialogBuilder extension library documentation

[michaellwest]

Scope

Promote the existing WIP draft (wip/dialog-builder-functions.md) to an official page. Update to reflect API reconciliation:

• Void returns on Add-DialogField/Remove-DialogField
• Show-Dialog renamed to Invoke-Dialog
• Export-DialogBuilderConfig renamed to ConvertTo-DialogBuilderJson
• Debug-DialogBuilder merged into Test-DialogBuilder -ShowDetails

Cross-reference existing interfaces/interactive-dialogs.md and appendix/common/read-variable.md. Include screenshots of dialog UI. Mark as "Introduced in SPE 9.0".

References

• Add Dialog Builder fluent API as opt-in extension module Console#1416
• Reconcile DialogBuilder API with SearchBuilder conventions Console#1429

Files

• Move/promote wip/dialog-builder-functions.md to modules/dialog-builder.md
• SUMMARY.md — Add under Modules