Collect documentation for standard GH-pages components

I understand the reasons outlined in [#651](https://github.com/github/pages-gem/issues/651#issuecomment-962031976) for not automatically upgrading to Jekyll 4.0 along with all the consequent dependencies, among them Liquid and Sass. The problem now is that as all of these projects continue to evolve, it becomes harder and harder to learn what capabilities are natively supported by GH-pages—one has to dig extra hard into any part of the the documentation to discover whether any particular claim is true for a GH-pages site. Consequently, GH-pages becomes harder and harder to use. **Ease-of-use is one of the things that has made GH-pages so incredibly popular and powerful over the years**, so not paying attention to the erosion of this quality is a threat to the system's continued relevance.

Personally I would prefer if GH would avoid breaking old websites by adding a "legacy jekyll" checkbox to repository configuration—frankly it seems easier than what I'm about to suggest—but failing that, I'd like to see GitHub maintain a site that collects all the documentation for versions of jekyll and the tools on which it's built that are actually supported.

Thanks for your attention.