docs: add application environment variables (#2577)

* docs: updated installation env vars and runtime values references

* docs: fix content

* docs: update content

* Update table of contents, and refactored docs.

* Fixed capitalization.

* batch update to fix readability

* refactored declarative env var

* Updated translation.

* Updated based on suggestions.

* Updated based on suggestions.

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Co-authored-by: yajing wang <413741312@qq.com>

docs/developer/develop/app-env-index.md

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+---
+outline: [2, 3]
+description: Learn how variables are injected during Olares app deployment, including declarative environment variables (.Values.olaresEnv) and system-injected runtime Helm values (.Values.*).
+---
+
+# Environment variables overview
+
+Olares apps use app-service to inject runtime context and configuration into the app's `values.yaml`. In Helm templates, you can reference these values via `.Values.*`.
+
+:::info Variables and Helm values
+In this document, "variables" mainly refer to Helm values. They are not automatically passed into container environment variables. If you need them inside containers, explicitly map them to `env:` in your templates.
+:::
+
+## How variables are injected
+
+Olares injects variables through two channels:
+
+- **Declarative environment variables**: The developer declares variables under `envs` in `OlaresManifest.yaml`. At deployment, app-service resolves and injects the values into `.Values.olaresEnv` in `values.yaml`.
+
+- **System-injected runtime variables**: Injected automatically by Olares at deployment time. No declaration is required, though some values are only available after you declare the relevant dependency, such as middleware.
+
+## Next steps
+
+1. [Declarative environment variables](app-env-vars.md): Field reference for the `envs` schema, including variable mapping and variable references.
+2. [System-injected runtime variables](app-sys-injected-variables.md): Full reference for all system-injected runtime variables.