x/pkgsite: swap ?GOOS=darwin to arm64? #72927
Description
By default, pkgsite renders for linux-amd64, but offers a few "popular" alternative GOOS/GOARCH combinations, currently linux/amd64, windows/amd64, darwin/amd64, and js/wasm.
I believe we have previously been hesitant to add more configurations due to additional processing cost, but I wonder if we should replace darwin/amd64 with darwin/arm64, given that the former is only declining in use due to no new darwin/amd64.
A one-off non-scientific look at https://storage.googleapis.com/prod-telemetry-merged/2025-03-15.json has ~90% of darwin reports from darwin/arm64.