Add SIGGRAPH 2023 redirect

Add a HTTP redirect for the SIGGRAPH hands-on class. It redirects to
https://siggraph.stuvel.eu/ so that I can easily update the website while
at SIGGRAPH itself.

web/project-website/content/siggraph2023/_index.md

@@ -0,0 +1,10 @@
+---
+title: SIGGRAPH 2023
+weight: 100
+
+geekdocCollapseSection: true
+geekdocNav: false
+geekdocHidden: true
+
+redirect: https://siggraph.stuvel.eu/
+---

web/project-website/layouts/partials/head/custom.html

@@ -1 +1,4 @@
 <script defer data-domain="flamenco.blender.org" src="https://analytics.blender.org/js/script.js"></script>
+{{- with .Page.Params.Redirect }}
+  <meta http-equiv="refresh" content="0; url={{.}}" />
+{{- end }}