Wiki page or Wiki site ?

SharePoint Online is undergoing some changes, so I wouldnt consider this the end all be all. That said a standard Team Site in SharePoint (before the Modern Team Sites) is essentially a Wiki Site. They already come with the Wiki Home Page enabled. A project site template does not come with the Wiki site/pages configuration and locks you in to web part pages with defined zones, where as the wiki pages are more flexibile from a layout perspective (without code). You then just create new pages (wiki pages) in the same library as the Home Page for the site. You still have the ability to activate all the other site features, although I would not recommend activating Publishing as that changes how pages work in your site and you move away from wiki style pages. You could also add separate Wiki libraries in your site. For instance my team has a site that has a lot of documents and some other main pages and then we have a specific wiki library created for knowledge base purposes.