The BASIC-256 documentation now serves under its own name at doc.basic256.org.
It is the same Docusaurus documentation site introduced in Beta 2, which previously lived on a GitHub Pages address. Moving it to a domain of its own means links to it stay valid regardless of where the files are hosted in future.
From Beta 3 onwards, the About box, Help → Online help and pressing F1 over a keyword all open the new address. The old GitHub Pages URL still resolves, so existing bookmarks are not broken.