add macos warning for potential ear damage, on downloads page

Hi,
after some requests on scsynth.org, I opened a PR to add a warning to SC's
website, below the download section for macOS.
Since even more than one person reported having suffered ear damage because
of this issue, I feel it's very important to address their concerns and
eventually protect future vulnerable users. Until we have a chance to fix
it (at least in 3.12), I think it's reasonable to put a warning message.

Comments and different wordings are very welcome!

PR: https://github.com/supercollider/supercollider.github.io/pull/220
Forum: https://scsynth.org/t/way-to-limit-maximum-output-volume/
Issue: https://github.com/supercollider/supercollider/issues/4894