Display pubspec-provided documentation link.

[isoos]

Some packages may have a site that hosts documentation separate from the main homepage. If your package has that, you can also add a documentation: field with that URL. If provided, a link to it is shown on your package’s page.

https://www.dartlang.org/tools/pub/pubspec#documentation

At the moment pub site ignores this setting. Source: dart-lang/site-www#801

[mit-mit]

How many packages actually do this?

[isoos]

How many packages actually do this?

(staging data) 230, out of which 210 are linking to external sites (e.g. not www.dartdocs.org or pub.dartlang.org). Considering that we have never displayed this value on the package page, I'd say it is significant.