Redirect devtools-next.vuejs.org to devtools.vuejs.org

[nielsbom]

The (v7) devtools were called "devtools-next" for a while. The docs site https://devtools-next.vuejs.org/ is still up. I think this is confusing, shall we redirect that to https://devtools.vuejs.org?

If this is the wrong repo for this kind of issue: let me know. If someone can point me where to make a PR: happy to help.