Add external feed trigger examples

[eddymoulton]

Adds additional docs to help users choose external feed triggers for their application.

I've added fairly minimal sections in existing kubernetes deployment pages to reference external feed triggers and their use cases + a whole new page to cover automatically updating Helm charts that you don't control.

Update to main external triggers page

Update to deploy raw yaml

Update to deploy Helm

Automatically tracking third party helm charts

[sc-75182]