doc: Document gazelle integration (#6894)

docs/gazelle.md

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+# Using Gazelle to Keep BUILD Files Updated
+
+Gazelle can automatically generate and update your Bazel `BUILD` files based on the imports in your source code. To keep your `BUILD` files in sync without manual intervention, configure Gazelle to run before each sync operation.
+
+## Setup Instructions
+
+1. **Add `gazelle_target` to Your Project View File**: Include the `gazelle_target` attribute in your project view file to specify the Gazelle target to run.
+
+   ```yaml
+   # Example project view file
+   directories:
+     src/
+   gazelle_target: //:gazelle
+   ```
+
+2. **Automatic Execution**: With the `gazelle_target` set, Gazelle will run automatically before every sync, updating your `BUILD` files according to your source code imports.
+
+## Example Project
+
+For a practical example, see this [Gazelle project setup](https://github.com/bazelbuild/intellij/tree/master/examples/go/with_proto).
+
+## Demo
+
+https://github.com/user-attachments/assets/b130bf56-49ec-46cc-810f-c831672e2601
+