Add a catchall wiki entry to redirect to for all broken links that cannot be restored (issues...)

[pschmitt]

We should create a wiki entry named null that explains why a specific link is dead.

And lists a few hints about where to maybe find the missing info regardless:

• google cache
• wayback machine
• etc

Also, it should link to "create new issue" ;)

Originally posted by @pschmitt in https://github.com/zdharma-continuum/zinit/pull/47#discussion_r748028208

TODOs:

• [x] Create wiki page: https://github.com/zdharma-continuum/zinit/wiki/%F0%9F%98%95-null (be aware of the emoji in the title :))
• [ ] Update links in this repo to point to it. The OG URL should be kept as is, only the underlying link should be altered,