Update Pipeline Documentation to remove DeployingCallbackAnnotation and PublishingCallbackAnnotation

[cdbrown2018]

The Building custom deployment pipelines documentation is outdated and contains references to DeployingCallbackAnnotation and PublishingCallbackAnnotation, neither of which exists anymore. The documentation should be updated to reflect the modern approach to creating custom pipeline steps and tapping into the publishing and deploying pipelines.

[IEvangelist]

Triage → @Copilot

Page: https://aspire.dev/deployment/custom-deployments/ → src/frontend/src/content/docs/deployment/custom-deployments.mdx

Scope: Remove references to DeployingCallbackAnnotation and PublishingCallbackAnnotation (both removed from microsoft/aspire). Replace with the current pipeline-step API for tapping into publishing and deploying. Confirm the modern surface by reading src/Aspire.Hosting/Publishing/ and src/Aspire.Hosting/Deploying/ on microsoft/aspire main, plus the 13.3/13.4 changelog entries.

Acceptance

• Zero remaining references to DeployingCallbackAnnotation / PublishingCallbackAnnotation on this page (or anywhere else under src/frontend/src/content/docs/).
• Replacement examples compile against the current public API and follow the existing tutorial structure.
• Cross-link to the relevant app-host/ or extensibility/ page where the new annotation/pipeline-step types live, if one exists.