[Docs] Document how to pull images from private registry

[SmritiSatyanV]

Description

Reference: Slack conversation To pull a private image from dockerhub, use ONE of the below:

1. Create a new service account and use this service account to launch the execution.
2. Use a default service account to launch the execution.
3. Create a pod that uses secret to pull image using custom pod template that is automatically added to every pod created by Flyte. Declare a pod template and communicate it with FlytePropeller. (Slack conversation)