Move Brim docs to a GitHub wiki

[philrz]

We've gotten into the habit of creating markdown docs in the repos next to the code. While some of these are commonly handled this way (README, CONTRIBUTING, CHANGELOG, and code_of_conduct), there's others that should likely move to a wiki (TROUBLESHOOTING, README-Windows). That wiki can surely be populated with more articles over time.

At some point we'll want to sync certain docs from the zq repo to the wiki as well (e.g. the ZQL docs) since Brim depends on these. However, as a first step we'll just get the wiki up & running with what's already in the Brim repo.

[philrz]

The wiki is now alive at https://github.com/brimsec/brim/wiki, with GitHub Actions that ensure it's always kept up to date with any content in the docs/ directory that's merged to master.