docs: document Docker gomplate configuration

[AkashKumar7902]

Overview

Closes #2645.

What this PR does / why we need it

• Documents that the official container entrypoint renders .yaml, .tpl, and .tmpl config arguments with gomplate before starting dex.
• Clarifies that this preprocessing is separate from dex's built-in $VAR config expansion.
• Notes that the Kubernetes example overrides the image entrypoint, so it uses dex config expansion rather than gomplate.

Special notes for your reviewer

Validation:

• go test ./cmd/docker-entrypoint -run TestRun -count=1
• go test ./cmd/dex -run "TestUnmarshalConfigWithEnv(NoExpand|Expand)$" -count=1
• git diff --check

[nabokihms]

@AkashKumar7902, hello and thank you for submitting the PR!

The README file is not the place for this kind of a doc. Please consider moving it to https://dexidp.io/docs/configuration/

[AkashKumar7902]

@nabokihms Thanks! You're right — I removed the README block. The PR now only adds a small inline comment in examples/k8s/dex.yaml explaining why gomplate doesn't apply when overriding the container command (which is a property of that specific example, not general doc).

I'll open a follow-up against the docs site to add the gomplate / DEX_EXPAND_ENV content under docs/configuration/. PTAL.