Expand documentation on how to use baseJobTemplate

[mitchnielsen]

Summary

I've noticed that lately we've gotten more questions about how to use base job templates, including:

• How to expose more environment variables
• How to expose environment variables from secrets
• How to set image pull secrets

Let's expand our docs a little to make usage more clear.

What we have so far:

• prefect-helm/charts/prefect-worker/README.md.gotmpl

  Lines 244 to 250 in 3acdb38

  	#### Updating the Base Job Template
  	If a base job template is set through Helm (via either `.Values.worker.config.baseJobTemplate.configuration` or `.Values.worker.config.baseJobTemplate.existingConfigMapName`), we'll run an optional `initContainer` that will sync the template configuration to the work pool named in `.Values.worker.config.workPool`.
  	Any time the base job template is updated, the subsequent `initContainer` run will run `prefect work-pool update <work-pool-name> --base-job-template <template-json>` and sync this template to the API.
  	Please note that configuring the template via `baseJobTemplate.existingConfigMapName` will require a manual restart of the `prefect-worker` Deployment in order to kick off the `initContainer` - alternatively, you can use a tool like [reloader](https://github.com/stakater/Reloader) to automatically restart an associated Deployment. However, configuring the template via `baseJobTemplate.configuration` value will automatically roll the Deployment on any update.

• prefect-helm/charts/prefect-worker/values.yaml

  Lines 113 to 128 in 3acdb38

  	## If unspecified, Prefect will use the default base job template for the given worker type. If the work pool already exists, this will be ignored.
  	## e.g.:
  	## baseJobTemplate:
  	## configuration: |
  	## {
  	## "variables": {
  	## ...
  	## },
  	## "job_configuration": {
  	## ...
  	## }
  	## }
  	## OR
  	## baseJobTemplate:
  	## existingConfigMapName: "my-existing-config-map"
  	baseJobTemplate: