Add external GSoC site links

conf.py

@@ -63,6 +63,20 @@ html_sidebars = {
 }
 
 html_theme_options = {
+    "external_links": [
+        {
+            "url": "https://summerofcode.withgoogle.com/help",
+            "name": "Help",
+        },
+        {
+            "url": "https://summerofcode.withgoogle.com/rules",
+            "name": "Rules",
+        },
+        {
+            "url": "https://summerofcode.withgoogle.com/terms",
+            "name": "Terms",
+        },
+    ],
     "header_links_before_dropdown": 4,
     "show_prev_next": True,
     "icon_links": [