Add Antigravity configuration instructions (#12730)

Added instructions for configuring Antigravity with the Dart and Flutter
MCP server, including installation steps for the necessary extensions.


## Presubmit checklist

- [x] If you are unwilling, or unable, to sign the CLA, even for a
_tiny_, one-word PR, please file an issue instead of a PR.
- [x] If this PR is not meant to land until a future stable release,
mark it as draft with an explanation.
- [x] This PR follows the [Google Developer Documentation Style
Guidelines](https://developers.google.com/style)—for example, it doesn't
use _i.e._ or _e.g._, and it avoids _I_ and _we_ (first-person
pronouns).
- [x] This PR uses [semantic line
breaks](https://github.com/dart-lang/site-shared/blob/main/doc/writing-for-dart-and-flutter-websites.md#semantic-line-breaks)
  of 80 characters or fewer.

---------

Co-authored-by: gemini-code-assist[bot] <176961590+gemini-code-assist[bot]@users.noreply.github.com>
Co-authored-by: Shams Zakhour <44418985+sfshaza2@users.noreply.github.com>

Author: 3 people

src/content/ai/mcp-server.md

@@ -62,6 +62,28 @@ This section provides instructions for setting up the
 Dart and Flutter MCP server with popular tools like Firebase Studio,
 Gemini CLI, Gemini Code Assist, Cursor, and GitHub Copilot.
 
+### Antigravity
+
+To configure [Antigravity][] to use the Dart and Flutter MCP server,
+use the [MCP Store][]:
+
+1. Locate the **Agent Side Panel** on the right. 
+  (It might be closed, so press Ctrl+Cmd+B to open it).
+2. Click the **`...`** (Additional options) menu-button.
+3. Select **MCP Servers**.
+4. Search for **Dart**.
+5. Select **Install**.
+
+It is also recommended to configure the Dart and Flutter Extensions:
+
+1. Open the View menu and select **Extensions**.
+2. In the Search Extensions field, enter **Flutter**.
+3. Click Flutter in the list, and select **Install**.
+   This installs both the Flutter and Dart extensions.
+
+[Antigravity]: https://antigravity.google/
+[MCP Store]: https://antigravity.google/docs/agent-side-panel
+
 ### Gemini CLI
 
 To configure the [Gemini CLI][] to use the Dart and Flutter MCP server,