Clarify docs about Helm valueFiles from the same Git repository

[dersimn]

There is a docs section about Helm value files from external Git repository explaining best-practice to include a values.yaml that's hosted in some external Git repository (other than the one where the ArgoCD Application is specified).

Please also add an example for a best practice to use a local values.yaml file within the directory structure of the ArgoCD Application:

|-- argocd
|   `-- nginx-app.yaml
`-- nginx
    `-- values.yaml

File argocd/nginx-app.yaml (in Git repo http://git.example.com/argocd/main-repo):

apiVersion: argoproj.io/v1alpha1
kind: Application
spec:
  sources:
    - repoURL: registry-1.docker.io/bitnamicharts
      chart: nginx
      targetRevision: 19.0.1
      helm:
        valueFiles:
          - $argo-main-repo/nginx/values.yaml
    - repoURL: http://git.example.com/argocd/main-repo  # this is the same we're reading this yaml from
      ref: argo-main-repo

Is this really the best practice to include a values.yaml from a sibling directory of the ArgoCD App itself or is there a better way without having to specify our own Git repo url again?
In case of migration etc. it's quite an easy fix to search&replace all the urls, but idk - sounds like a clear "don't repeat yourself" violation to me.

[crenshaw-dev]

I'm not sure it makes sense for Argo CD to be opinionated about this. It's fine to keep a values.yaml alongside the Argo CD app. It's also fine to store it elsewhere. If you have specific pros/cons to list, we could probably add those. But I'm not sure there's a firm best practice here.

[dersimn]

But how to achieve this without hard-coding the GitHub URL in the YAML? Let‘s say I want to keep the GitHub independent from the target platform and specify the Gut repo only once (in the app-of-apps main-yaml)