[core] Add support for setting environment variables (#5635)

* Add env docs

* Update content/components/esphome.md

Author: swoboda1337

content/components/esphome.md

@@ -52,6 +52,9 @@ Advanced options:
 - **platformio_options** (*Optional*, mapping): Additional options to pass over to PlatformIO in the
   platformio.ini file. See [`platformio_options`](#esphome-platformio_options).
 
+- **environment_variables** (*Optional*, mapping): Environment variables to set during the build process.
+  See [`environment_variables`](#esphome-environment_variables).
+
 - **includes** (*Optional*, list of files): A list of C/C++ files to include in the (auto-generated) `main` file.
   The paths in this list are relative to the directory where the YAML configuration file is located or `<...>` includes.
   See [`includes`](#esphome-includes).
@@ -192,6 +195,27 @@ esphome:
     board_build.f_flash: 80000000L
 ```
 
+{{< anchor "esphome-environment_variables" >}}
+
+## `environment_variables`
+
+With the `environment_variables` option, you can set environment variables that will be available during the
+compilation process. This is useful when you need to pass configuration to build scripts, custom libraries,
+or PlatformIO build environments.
+
+```yaml
+# Example configuration entry
+esphome:
+  # ...
+  environment_variables:
+    HTTP_PROXY: "http://user:pass@10.10.1.10:3128/"
+    PLATFORMIO_SETTING_ENABLE_PROXY_STRICT_SSL: "false"
+```
+
+> [!NOTE]
+> Environment variables set here are only available during the build process. They do not affect
+> the runtime behavior of your device.
+
 {{< anchor "esphome-includes" >}}
 
 ## `includes`