dartdoc CI for building SDK docs needs to use git source

[srawlins]

Recently @athomas changed how these header/footer files are used in the SDK's dartdoc options at https://dart-review.googlesource.com/c/sdk/+/354886. He triple checked with @devoncarew and I, but dartdoc's CI was not ready for it 😅

This is totally valid, it just means we cannot test dartdoc generating a specific Dart SDK by relying on a published version of the SDK; we should just rely on a git checkout of the SDK, as this is how dartdoc builds the docs for api.dart.dev.

[srawlins]

For now, I was able to just submit some header and footer stubs. I think this is in a stable place.