Document native Helm post-renderer behavior

[vigneshrajsb]

Summary

• document native Helm post-renderer configuration and service override behavior
• remove duplicated native Helm sections that overlap with the template variables guide
• trim the Helm schema page to the common nativeHelm.postRenderer overrides
• add the missing supported service template variables to the dedicated template variables doc

Why

This branch documents the native Helm post-renderer changes from the app and tightens the docs so they match current behavior more closely.

It also removes duplicated or unsupported examples from the native Helm guide, including unsupported _dockerTag references, and keeps template-variable details in a single source of truth.

Validation

• bun run build
• repo-wide grep for GoodRx-internal references in src/pages/docs

Notes

• The repo-wide scrub did not turn up any new GoodRx-internal domains or account-specific references in the touched docs.
• Existing global_config storage references were left in place intentionally.