[housekeeping] Add docs about tpl function used for rendering `template` fields in such charts as `raw`

[MaxymVlasov]

Thank you so much for your work!
A have a few suggestions, based on my current struggle, how this project can get much wider adoption.

Is your feature request related to a problem? What would you like to improve?

Docs. Without good docs, charts are barely used by outsiders, as there is too much tribal knowledge which I can't easily access.

Describe the solution you'd like

Write/generate docs about how to use existing functionality, especially templates in raw

Additional context

All what I want - is just provide value inside raw.templates.[]. using some helm(?) interpolation.

But all attempts failed:

1. Based on raw README, I created a minimum reproducible configuration:

image:
  registry: docker.io

raw:
  templates:
    - |
      apiVersion: v1
      kind: Service
      metadata:
        name: foo
        namespace: bar
        labels:
          exampleLabel: {{ .Values.image.registry }} 

And it just breaks ArgoCD deployment:

2. If use next value, directly from template

exampleLabel: \{\{ include "lib.utils.globals.image" (dict "image" .Values.image "context" $ ) }}

it will not render at all (will just literally \{\{ include "lib.utils.globals.image" (dict "image" .Values.image "context" $ ) }} inside input parameters)

3. I tried to use

exampleLabel: {{ include "lib.utils.globals.image" (dict "image" .Values.image "context" $ ) }}

but this time it says that there is no such library as lib.utils.globals.image, and I can't find out any docs about this as well. Is I should install something? Or I need to create in values.yaml something like this:

helm-charts/charts/manifests/templates/utils/_common.tpl

Lines 66 to 74 in c3d711b

	{{/*
	Sprig Template - CommonLabels Wrapper. Adds Bundlename to CommonLabels, if defined.
	*/}}
	{{- define "bedag-lib.utils.common.commonLabels" -}}
	{{- include "lib.utils.common.commonLabels" . | indent 0 }}
	{{- if .bundlename }}
	app.kubernetes.io/bundle: {{ .bundlename }}
	{{- end }}
	{{- end -}}

I am not sure.

3.1. Also

helm-charts/charts/raw/values.yaml

Lines 30 to 31 in c3d711b

	# templates -- Define templates which will be executed using the `tpl` function
	templates: []

says that there some tpl function involved, but I can't find it, nor docs for how it works