docs: render Rust API link as docs.rs icon next to GitHub

Move the Rust API docs entry from external_links to icon_links and use
the fa-brands fa-rust gear mark. Now sits next to the GitHub icon in
navbar_end with matching visual weight instead of a wider text link.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: timsaucer

docs/source/conf.py

@@ -132,16 +132,17 @@ def setup(sphinx) -> None:
             "url": "https://github.com/apache/datafusion-python",
             "icon": "fa-brands fa-github",
         },
+        {
+            "name": "Rust API docs (docs.rs)",
+            "url": "https://docs.rs/datafusion/latest/datafusion/",
+            "icon": "fa-brands fa-rust",
+        },
     ],
     "external_links": [
         {
             "name": "Examples",
             "url": "https://github.com/apache/datafusion-python/tree/main/examples",
         },
-        {
-            "name": "Rust API",
-            "url": "https://docs.rs/datafusion/latest/datafusion/",
-        },
     ],
     "secondary_sidebar_items": [],
     "collapse_navigation": True,