Create proper documentation for dartx

[komape]

As far as I know dartx's content is only documented in code and in README.md. Would be nice to create some real docs like dart itself does. I've never done this but I found dartdoc which looks promising. Could be set up as a GitHub Page.

[simc]

@komape Most (if not all) of the extension methods have docs. Pub.dev automatically generates them: https://pub.dev/documentation/dartx/latest/dartx/dartx-library.html

[komape]

@leisim But for every method I have to open the seperate extension doc. Is there a way to show all docs of extensions that extend the same class in one doc (besides README.md)?