header: link donate button to getmoneros contrib. page

getmoneros contributing page is fully-featured (contains both the Monero and Bitcoin donation addresses and the QR codes work. It has translated strings)

*when -site has a 'donate' anchor the link will take people directly to the Donate section

example when anchor is added in PR 2287: https://deploy-preview-2287--barolo-time-757cf9.netlify.app/get-started/contributing/#donate