adds docs around web preview (#1266)

# Motivation

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# Content

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# Testing

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# Please check the following before marking your PR as ready for review

- [ ] I have added tests for my changes
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needed

Author: kopekC

docs/sandboxes/environment-variables.mdx

@@ -25,6 +25,7 @@ The following environment variables are typically available within Codegen sandb
 | `NPM_CONFIG_YES`                  | `"true"`                                      | Configures npm to automatically answer "yes" to prompts.                                                                                    |
 | `PIP_NO_INPUT`                    | `"1"`                                         | Instructs pip (Python's package installer) to operate in non-interactive mode.                                                              |
 | `YARN_ENABLE_IMMUTABLE_INSTALLS`  | `"false"`                                     | Disables Yarn's "immutable installs" feature, allowing `yarn install` to modify the lockfile.                                               |
+| `CG_PREVIEW_URL`                  | _Dynamic (preview URL)_                       | Contains the URL where the web preview will be served. This is automatically set when Web Preview is configured.                            |
 
 <Note>
   The values for variables like `NVM_DIR`, `PATH`, `NVM_BIN`, and `NODE_VERSION`

docs/sandboxes/web-preview.mdx

@@ -57,6 +57,13 @@ bundle exec rails server -b 127.0.0.1 -p 3000
   Variables](./environment-variables).
 </Tip>
 
+<Tip>
+  The `CG_PREVIEW_URL` environment variable is automatically set and contains
+  the URL where your web preview will be accessible. Use this in your
+  application code when you need to reference the preview URL programmatically
+  (e.g., for CORS configuration, webhooks, or generating absolute URLs).
+</Tip>
+
 <Note>
   The web preview is intended for development and debugging purposes. The server
   is typically only accessible while the agent run is active or for a short