docs: how to use inline Helm values in ArgoCD manifests

[jstaf]

Description

This adds documentation on how to use argocd-vault-plugin with Helm values inline in the ArgoCD application manifest (similar to the --values-literal-file option or declarative setup here https://github.com/argoproj/argo-cd/blob/cd497c4eec84a47040816a0b04361005c3ce0ee8/docs/operator-manual/application.yaml#L39-L52).

I also changed the example argocd-cm.yml manifest to include $ARGOCD_APP_NAME to match the examples from the docs.

Fixes: #304 and also #266 (but only for the CLI workflow).

Checklist

Please make sure that your PR fulfills the following requirements:

• Reviewed the guidelines for contributing to this repository
• The commit message follows the Conventional Commits Guidelines.
• Tests for the changes have been updated
• Docs have been added / updated
• Optional. My organization is added to USERS.md.

Type of Change

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

[CLAassistant]

All committers have signed the CLA.

[werne2j]

Thanks for doing this @jstaf!

[jkayani]

Thanks! I didn't realize process substitution could be of use here