Unify look with flutter API docs

[guidezpl]

The look of Dartdoc is a bit outdated compared to Flutter API docs. They would benefit from a fresh coat of paint to match:

• update color scheme
• update font(s)
• add view source code buttons
• fix search bar
• consider removing documentation page from breadcrumbs, since this just replicates the pub.dev package page, and instead link directly to the first library

Brightness	Flutter API docs	Pub dartdoc
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[sigurdm]

cc @szakarias