Nobody would call it an "SDK" but I find the Emacs/Elisp documentation to be excellent both on the docstring level and the manuals. It has been refined over sev

Nobody would call it an "SDK" but I find the Emacs/Elisp documentation to be excellent both on the docstring level and the manuals. It has been refined over several decades, it is thorough, well written and very easy to use from the comfort of Emacs itself.