x/pkgsite: create a way for users to opt-in to experiments before they are fully rolled out

[julieqiu]

As part of the process for rolling out new features, certain features are put behind experiment flags:

https://pkg.go.dev/golang.org/x/pkgsite/internal/?tab=doc#pkg-variables

Users may be interested in opt-ing in to use these "experimental" features, ahead of the feature being fully rolled out. We should consider creating a UI that would enable users to do so from pkg.go.dev.

This would enable us to get early feedback, before features fully go live on the site.

[julieqiu]

This is available via https://beta.pkg.go.dev.