Update quickstart.md to include docs folder

[kolibril13]

sphinx projects are often in the "docs folder",
and also the later part of the quick start makes this assumption:
https://myst-nb.readthedocs.io/en/latest/quickstart.html

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[kolibril13]

And another idea:
I use virtual python environments, and
which sphinx-build points by default to
/opt/homebrew/bin/sphinx-build.
And that causes the error:

Extension error:
Could not import extension myst_nb (exception: No module named 'myst_nb')

In order to point to the correct sphinx-build in my virtual environment, I had to run
pip install sphinx once, and then it worked without problems

[choldgraf]

Seems reasonable to me, thanks !

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