Update other examples to link to files.

Author: pauleveritt

src/psc/gallery/examples/antigravity/index.md

@@ -1,5 +1,7 @@
 ---
 title: xkcd Antigravity
 subtitle: We can fly!
+linked_files:
+  - antigravity.py
 ---
 Based on the [xkcd antigravity](https://xkcd.com/353/)

src/psc/gallery/examples/hello_world_py/index.md

@@ -1,5 +1,9 @@
 ---
 title: Hello World Python
 subtitle: The hello world example, but in a .py file.
+linked_files:
+  - hello_world.py
+  - hello_world.css
+  - hello_world.js
 ---
 The *body* description.