Improve Readme and doc

I really appreciate this package but I find the documentation difficult for newcomers:
- Various discrepancies in the name of the parameters between Readme, methods' signatures and Doc. For example these 3 names are used for the same parameter: `canExecute`, `restriction`, `registration`. I saw that this problem is mentioned in issue [#47 Nullability support](https://github.com/fluttercommunity/rx_command/issues/47#issuecomment-824623306) which cite another different name, `constraint`.
- The Readme seems to be aimed at those who already have some familiarity with this architecture, which is understandable; in the blog https://www.burkharts.net/apps/blog there are articles suitable for newcomers, but they are not up-to-date (for example, reference is made to the Observable of RxDart, which from version 0.23 directly uses the Stream). Not that this is a problem, I just guess that's why these guides weren't promoted more in the Readme.

I am inexperienced and unfamiliar with git but am available to help update the documentation. Personally, the guide that got me aficionado of this package was the blog article https://www.burkharts.net/apps/blog/rxvms-foundations-rxcommand-and-getit/, together with the two articles of the same series preceding it (which is much more complete than "Making Flutter more Reactive" mentioned in the Readme). One idea could be to update this series of articles to the current RxCommand and RxDart Api and make it available in the wiki of this repo.

Beginne mit der README des Repositorys und vergleiche ihre Parameternamen mit den Methodensignaturen und der Dokumentation. Sieh dir anschließend Issue #47 und die verlinkte Blogserie zu RxCommand und RxDart an. Kläre den Umfang der Dokumentation und wähle einen konsistenten Namenssatz; abgeschlossen ist die Aufgabe, wenn die Anleitung für neue Mitwirkende die aktuelle API widerspiegelt und das vorgeschlagene Wiki-Material auf dem neuesten Stand ist.

Vom Indexierungsmodell aus dem Issue-Text verfasst.