As of early 2020, Roll20 Help Center is the location of all official Roll20 Documentation. Roll20 have added Help Center links to the top wiki pages that previously served as officially maintained pages by Roll20, and copied the content of the pages to the Help Center, where they have continued to update some.

The Roll20 Wiki stays as a Community Wiki where users can share and update info on various aspects of Roll20, see the About-page for more details.

The Help Center can be seen as having better documentation on newer Roll20 features released from 2020 onward, while some some sections of the Community Wiki will remain better maintained by the users, such as documentation related to Building Character Sheets, and pages for popular Roll20 sheets such as D&D 5E by Roll20.

The Community Wiki is also more likely to address existing issues or quirks on some features, and link to relevant recent discussions on the topic, as well as containing bunch of user-created general guides covering topics that might fall through the cracks for the Official Documentation.

So when looking up info about Roll20, it's a good idea to check both the Community Wiki and the Roll20 Help Center