Introduce golang import namespace "knative-sandbox.dev" ?

[rhuss]

Currently (as pointed out and briefly discussed in https://github.com/knative/website/pull/187/files#r485484109) every knative and knative-sandbox repo is unconditionally added to the knative.dev golang vanity domain. I think the original motivation was to make migration of various repos originally hosted as part of the knative GitHub to the knative-sandbox easier. But on the other hand, and if I remember correctly, knative-sandbox was never intended to be as an incubation stage, so not necessarily every sandbox project is a target for eventual inclusion in knative.

So I suggest to introduce a new goimport vanity domain knative-sandbox.dev to make that clear, if its not an unjustifiable amount of work. For this, to work we would need to

• [ ] Register domain knative-sandbox.dev (if not already done)
• [ ] Create a new Github repo knative/sandbox-website
• [ ] Add a trimmed down clone of knative/website that only contains the redirects
• [ ] Adapt issue template in knative/community to point to this new location for the redirects

For example, I would see all kn plugins in this namespace, but maybe also other sandbox repos, like eventing sources. However, we could also leave it as it but then might confuse people who want to navigate from a go-import to the GitHub repository behind this.

[rhuss]

no need to change the current behaviour, so that we can keep import paths stable in case something is moving from knative-sandbox to knative.