add Connections crosslinks to Existing app tutorial

[willtsai]

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Description

Small change to Existing application tutorial that adds crosslinks to the Radius Connections materials.

Issue reference

This addresses some feedback collected from the community during office hours.

[Reshrahim]

Thanks for updating this. The office hours user did not know how to add connections to existing k8s manifests The connections overview doesn't cover the annotation to use to add connections. I think we should add a simplified version of step 6 from helm tutorial in addition to crosslinking.

[willtsai]

Thanks for the suggestion @Reshrahim - I've updated the cross linking to specifically address the community feedback you've mentioned. However, I think there is a separate issue where the Annotations features aren't documented beyond the tutorials, so I've created this issue to track that work: #1117

[Reshrahim]

sounds good!