References to missing docs

[kalafut]

PR #4129 removed the FAQ and some other docs and I've bumped into a couple of references to them (I'm sure there are more):

• Error advice: Potential outputs must be unique across all builders. See https://github.com/dart-lang/build/blob/master/docs/faq.md#why-do-builders-need-unique-outputs
• The links in the "NOTE:" section of https://pub.dev/packages/build_runner are broken.

[davidmorgan]

Thanks!

I deleted most of the FAQs since they are very old--I'd like to get an idea what problems people are actually running into and build up the docs again based on that :)

So, sent a PR removing the link.

For pub.dev/packages/build_runner, that will be addressed next time I release the package, which will be soon :) you can see the upcoming version here

https://github.com/dart-lang/build/tree/master/build_runner

[q0w]

This page should be also updated https://dart.dev/tools/build_runner#more-information
dart-lang/site-www#6837