Support calling from outside of the book directory (argument PATH_SOURCE)

[paugier]

With Jupyter Book <2, it was possible to have a layout for which the content of the book is in a directory, which gives something like:

$ ls
book  dir0  dir1  Makefile  pdm.lock  pyproject.toml  README.md

and one can call jupyter-book build book, where book is the PATH_SOURCE

$ jupyter-book --version
Jupyter Book      : 1.0.3
External ToC      : 1.0.1
MyST-Parser       : 2.0.0
MyST-NB           : 1.1.2
Sphinx Book Theme : 1.1.3
Jupyter-Cache     : 1.0.1
NbClient          : 0.10.1

$ jupyter-book build -h
Usage: jupyter-book build [OPTIONS] PATH_SOURCE

It seems that it is not currently possible with mystmd and Jupyter Book 2.

$ myst build -h
Usage: myst build [options] [files...]
$ jupyter-book --version
v2.0.0-a3
$ jupyter-book build -h
Usage: jupyter-book build [options] [files...]