add an auto-roll of dartdoc into the SDK

[jcollins-g]

Having dartdoc roll automatically into the SDK would save a bit of engineering time on every release or important PR. If an auto-roll is not appropriate, at least a semi-automatic system (run one command/push one button, it does everything) would be nice.

[devoncarew]

FYI that we have two tools here that would help: https://github.com/dart-lang/sdk/wiki/Adding-and-Updating-Dependencies#rolling-dependencies.

Generally we use the batch rolling tool (<sdk>/tools/rev_sdk_deps.dart) to roll all package deps ~twice a week, including dartdoc.

If you want to roll more frequently, or roll a recent commit, you can instead run dart tools/manage_deps.dart bump third_party/pkg/dartdoc. That will roll dartdoc to the latest and optionally create a CL for you.