Migrate wiki and update links

This migrates the uxr wiki into checked in files, and updates all old wiki links

I'll update the original wiki pages to point to these new places after this lands

Pre-launch Checklist

[x] I read the Contributor Guide and followed the process outlined there for submitting PRs.
[x] I read the Tree Hygiene wiki page, which explains my responsibilities.
[x] I read the Flutter Style Guide recently, and have followed its advice.
[x] I signed the CLA.
[x] I listed at least one issue that this PR fixes in the description above.
[x] I updated/added relevant documentation (doc comments with ///).
[x] I added new tests to check the change I am making, or this PR is test-exempt.
[x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel on Discord.