Add custom gist shortcode to replace deprecated Hugo built-in (#16403)

Creates a custom gist.html shortcode to replace Hugo's deprecated
built-in gist shortcode, eliminating the deprecation warning.
The custom implementation maintains full compatibility with all
34 existing gist usages across blog posts.

Fixes #16396

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-authored-by: Claude <[email protected]>

Author: jkodroff

layouts/shortcodes/gist.html

@@ -0,0 +1 @@
+<script src="https://gist.github.com/{{ index .Params 0 }}/{{ index .Params 1 }}.js{{if len .Params | eq 3 }}?file={{ index .Params 2 }}{{end}}"></script>