Add docs on where to specify imagePullSecrets for all products

[tnederlof]

We should add some information in the READMEs for each product showing which values to specify to set the imagePullSecrets for private repo authorization:

image.imagePullSecrets for Workbench, Connect and Package Manager
launcher.templateValues.imagePullSecrets for Workbench and Connect

[edavidaja]

• Connect: https://docs.posit.co/helm/examples/connect/container-images/private-images.html
• Package Manager: https://docs.posit.co/helm/examples/package-manager/container-images/private-images.html
• Workbench: https://docs.posit.co/helm/examples/workbench/container-images/private-images.html