[ENH] jupyter-book config sphinx --output-dir

[parmentelat]

this PR adds an option to jupyter-book config sphinx to select another folder for resulting conf.py

my use case:
I have a rather big repo that ends up into 2 separate books
https://github.com/flotpython/exos
(admittedly this should be cut into pieces but that's another story...)

the jupyter-book stuff (config and tocs) are stored in a separate folder jb/ for clarity
I can produce the 2 books locally easily enough

but when trying to move to readthedocs.io,
I made many various attempts on how to layout things and call jb config sphinx but I'm running into a wall
and it looks like my life would be way easier if the resulting conf.py ended up at the toplevel

so I figured, this change here looks innocuous enough, and it would help greatly to achieve this rtd migration, that I can't seem to provide otherwise

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[parmentelat]

Hiya
is there anything I can do to improve the chances of this PR ?

[agoose77]

@parmentelat no! Let me get around to this later today.

[parmentelat]

trying to rekindle this, please let me know how to interpret the above results, and what I could do the help make it through