Move to Netlify (#1683)

* Remove GitHub Pages publish script

* Redirect katex.netlify.com to katex.org

* Redirect function-support using _redirects

* Redirect docs to the first documentation and docs/en/ to docs/

* Include HSTS includeSubDomains and preload directive

* Cache static image assets

website/static/_headers

@@ -0,0 +1,5 @@
+/*
+  Strict-Transport-Security: max-age=31536000; includeSubDomains; preload
+
+/img/*
+  Cache-Control: public, max-age=31536000