Add a donate button to landing page

Author: jmunroe

portal/index.md

@@ -17,6 +17,9 @@ An education and training hub for the geoscientific Python community
 
 Project Pythia is hosting a Cookbook Cook-Off June 11-14, 2024. {button}`Cookbook Cook-Off<https://projectpythia.org/pythia-cookoff-2024>`
 
+Donate to support Project Pythia!
+{button}`Donate now<posts/fundraiser.md>`
+
 +++ { "kind": "justified"}
 
 ## What is Project Pythia?