docs: address maintainer feedback on email template customization

[Gitjay11]

Summary

This is a follow-up PR to address maintainer feedback from PR #6313 regarding the email template customization documentation.

The changes include:

• Encoding Static Parameters: Added a recommendation and updated structural examples to explicitly hardcode non-variable paths (like the base URL for invitations and sign-in) alongside the {{.InvitationCode}}, rather than relying on fully dynamic variables like {{.InvitationURL}} or {{.SignInURL}}. This allows operators to easily configure default Stacklok-specific variables (like Terms and Privacy URLs).
• Helm Deployment Updates: Added a dedicated sub-section demonstrating how operators using Helm can inject their custom templates by utilizing the deploymentSettings.extraVolumes and deploymentSettings.extraVolumeMounts values.
• Debugging Constraints: Rewrote the debugging instructions. Since the Minder container is a distroless image built with ko (which lacks a shell and core utilities like ls and cat), using kubectl exec is not viable. The guide now encourages users to verify configurations using kubectl describe deployment or check the underlying OCI artifact using the external dive tool.

No new dependencies are introduced.

Follow-up to #6313

Testing

• Verified that the Markdown file renders correctly and the styling matches the layout standards.
• Confirmed the YAML syntax inside the newly added Reference via Helm Chart section is correct and aligns with Minder's defined deploymentSettings schemas.
• Reviewed the dive command and kubectl describe deployment debug instructions to verify they represent the accurate workflows for testing distroless pods and OCI volume mounts.

[coveralls]

coverage: 59.387% (-0.003%) from 59.39% — Gitjay11:docs/email-template-customization-followup into mindersec:main