Add xAI to ChatModelTabs drop down (#30028)

Thank you for contributing to LangChain!

- [ ] **PR title**: "docs: add xAI to ChatModelTabs"

- [ ] **PR message**:
- **Description:** Added `ChatXAI` to `ChatModelTabs` dropdown to
improve visibility of xAI chat models (e.g., "grok-2", "grok-3").
    - **Issue:** Follow-up to #30010 
    - **Dependencies:** none
    - **Twitter handle:** @tiestvangool 

If no one reviews your PR within a few days, please @-mention one of
baskaryan, efriis, eyurtsev, ccurme, vbarda, hwchase17.

---------

Co-authored-by: Chester Curme <chester.curme@gmail.com>

docs/src/theme/ChatModelTabs.js

@@ -211,6 +211,13 @@ ${llmVarName} = ChatWatsonx(
       apiKeyName: "DATABRICKS_TOKEN",
       packageName: "databricks-langchain",
     },
+    {
+      value: "xai",
+      label: "xAI",
+      model: "grok-2",
+      apiKeyName: "XAI_API_KEY",
+      packageName: "langchain-xai",
+    },
   ].map((item) => ({
     ...item,
     ...overrideParams?.[item.value],