Zui-fy the Brim wiki

[philrz]

At the same time that we make the transition of the app name from "Brim" to "Zui", the wiki currently at https://github.com/brimdata/brim/wiki needs to mention the transition and also start referring to the app by its new name.

A stretch goal would be to simultaneously transition the wiki content into a separate docs site like we've done with https://zed.brimdata.io/. Once we've done this, we'll need to figure out how it fits in with what's already at that site, e.g., will there be a wholly separate page for https://zui.brimdata.io/ and a link between the two, or both land at the same page but with links to both sets of docs? Current consensus at a recent team meeting was something more like the latter, but final decision TBD.

[philrz]

Note to self: When docs are at their post-GitHub-wiki home, link to them from the Welcome page (#2387).

Also the link in #1497.

Also change the link in the New Issue template, as it currently points to the GitHub wiki.

[philrz]

Along with today's release of Zui v1.0.0, the former Brim wiki that's now at https://github.com/brimdata/zui/wiki has been replaced with a simple pointer to the new site https://zui.brimdata.io/docs. Regarding the other follow-on items mentioned in comments above, there are open issues for #1497 and #2704 that will now be pursued, and the New Issue templates also link to the new docs site.