Change default dartdoc resources 6.0.1 -> 6.1.4

Make the assets path configurable

Switch our reference dartdoc to version 6.1.5

Switch our reference dartdoc to version 6.0.1

Make BranchUpdate slightly more open

Require a minimum of Dart SDK 2.17.0

Stabilise dependencies ahead of the 0.5.0 release

Remove hardcoded branch names from InPlaceBranchUpdate

Smooth out the logging logic in BranchUpdate

A better abstraction for updating branches with docs