site/docs: regenerate static website using Hugo

For now it's manual:

$ cd site
$ rm -rf docs/*
$ hugo

I will explore making this automatic using a GitHub Action.

site/docs/global-sustainability-issue/global-sustainability-issue.txt

@@ -0,0 +1,12 @@
+Child rights and wellbeing
+Climate change
+Consumers and supply chains
+Forests and other ecosystems
+Freshwater and oceans
+Health and wellbeing
+Livelihoods
+Participant costs and benefits
+Pesticides, fertilizers and soil
+Plant and wildlife conservation
+Rights of indigenous peoples and local communities
+Wages and workers' rights