adds docs around web preview (#1266)

# Motivation <!-- Why is this change necessary? --> # Content <!-- Please include a summary of the change --> # Testing <!-- How was the change tested? --> # Please check the following before marking your PR as ready for review - [ ] I have added tests for my changes - [ ] I have updated the documentation or added new documentation as 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