The extension should set `docfile` and `docdir` instead of `base_dir`

[ggrossetie]

Original discussion: https://asciidoctor.zulipchat.com/#narrow/channel/279642-users/topic/Base.20directory.20and.20IDE/near/421004097

base_dir is basically a design bug in Asciidoctor (or perhaps we can say for niche situations). It should never be used unless you absolutely know why you are using it. Setting it breaks relative includes. If docdir is set, it should be capable of doing all the work that is necessary.
-- Dan Allen

I don't think I set it, browsed the code and didn't find it. I set only docfile (current file to be rendered for the preview) and docdir (the folder where the file is located).
-- Alexander Schwartz

That's the correct way.
-- Dan Allen