docs: update addons secret only gets injected to main container

[KollaAdithya]

Update the documentation to reflect that only the main container will be automatically injected secrets from the exports of an addon stack but not sidecar containers.

https://aws.github.io/copilot-cli/docs/developing/addons/workload/#writing-the-outputs-section

Here is a workaround using yamlpatch discussed in this issue to inject into sidecar containers.