[SPARK-52107] Add workflow_dispatch to pages.yml to support manual trigger

### What changes were proposed in this pull request?

This PR aims to add `workflow_dispatch` to `pages.yml` to support manual trigger

### Why are the changes needed?

To control the website.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

Manually after merging.

### Was this patch authored or co-authored using generative AI tooling?

No.

Closes apache#192 from dongjoon-hyun/SPARK-52107.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

.github/workflows/pages.yml

@@ -23,6 +23,11 @@ on:
   push:
     branches:
       - gh-pages
+  workflow_dispatch:
+    inputs:
+      branch:
+        required: true
+        default: '["gh-pages"]'
 
 jobs:
   docs: