Add Plausible goal for referrals to GitHub Sponsor page

[cassidyjames]

Probably want to know how many folks we're linking to the Sponsors page, for example.

[btkostner]

Can't we just see this from https://plausible.io/builds.elementary.io?page=%2Fsponsor

[cassidyjames]

@btkostner but it'd be useful to know how many people actually click through and visit GitHub Sponsors externally when prompted by the builds site.