Migrate devtools wiki & update old flutter wiki links

This migrates the home and triage page of the devtools wiki into checked in files, and updates all old references to flutter/flutter/wiki

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 there is a reason for not adding tests.

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