Referring to API docs

TODO: Add info on setting these variables.

Use the site.dart_api and site.data.pkg-vers.SDK.channel config variables to make sure you point to the right version of API docs. Some examples:

• https://api.dartlang.org/stable/1.24.3/index.html
  -> {{site.dart_api}}/{{site.data.pkg-vers.SDK.channel}}
  which resolves to https://api.dartlang.org/stable or https://api.dartlang.org/dev.

• https://api.dartlang.org/stable/1.24.3/dart-core/dart-core-library.html
  -> {{site.dart_api}}/{{site.data.pkg-vers.SDK.channel}}/dart-core
  which resolves to https://api.dartlang.org/stable/dart-core or https://api.dartlang.org/dev/dart-core
  (Note: Drop the /libname-library.html at the end of the URL.)

• https://api.dartlang.org/stable/1.24.3/dart-core/Function-class.html
  -> {{site.dart_api}}/{{site.data.pkg-vers.SDK.channel}}/dart-core/Function-class.html
  which resolves to https://api.dartlang.org/stable/dart-core/Function-class.html or https://api.dartlang.org/dev/dart-core/Function-class.html