https://stardewvalleywiki.com is the official wiki for the game, hosted by ConcernedApe since early 2021, and editable by anyone. Prior to CA taking over hosting the wiki site, it was hosted by Chucklefish, the former publisher of Stardew. When ownership of the wiki changed, certain aspects of the wiki couldn't be transferred due to privacy requirements. Chucklefish set up
https://stardewcommunitywiki.com as an archive copy of the wiki at the time it was transferred. This allows people to go back and reference old "discussion" pages and old user pages. A good comparison of the difference is to go to a page on both sites and click on the "discussion" tab. For example:
https://stardewvalleywiki.com/Talk:Penny shows people discussing the page for Penny, beginning in 2021 when this copy of the wiki began, while
https://stardewcommunitywiki.com/Talk:Penny includes discussion of the page from 2016 to 2021.
While a lot of older discussion would never need to be checked, it was useful to have the archived wiki right after the transfer happened, as some pages had active discussions occuring, and the archive ensured people could check what had been discussed so far and continue the discussion from there on the new site.
Here are ConcernedApe's announcements from 2021:
Upcoming Wiki Transfer - January 8th, 2021
Stardew Valley Wiki Ownership Change - February 23rd, 2021