Add a "contribute on GitHub" to the website

It's non obvious for users of the site to figure out how to contribute things by reporting bugs or creating PRs to fix them.  Let's add a link to the github page in the header to make that easier for them.

See for example this email, that was first on pgsql-bugs https://www.postgresql.org/message-id/78c6b9661e1f88988b9b4fcd29b42751@postgrespro.ru