Add note about failure modes when using sphinx-build

[elide-b]

Running the documentation build from different working directories, or placing the build directory inside of the source directory causes strange failures (see below), which for most Sphinx builds is the default layout, so I've added a note to clarify that for the NEST doc build that is not a suitable option ✨

Notebook error:
CellExecutionError in _build/_downloads/0a77cecc347c93bf5d339ab292f0f3ad/eprop_supervised_classification_neuromorphic_mnist.ipynb:
------------------
pixels_blocklist = np.loadtxt("./NMNIST_pixels_blocklist.txt")

...

File ~/wsl-git/nest-simulator/.venv/lib/python3.12/site-packages/numpy/lib/_datasource.py:529, in DataSource.open(self, path, mode, encoding, newline)
    526     return _file_openers[ext](found, mode=mode,
    527                               encoding=encoding, newline=newline)
    528 else:
--> 529     raise FileNotFoundError(f"{path} not found.")

FileNotFoundError: ./NMNIST_pixels_blocklist.txt not found.

[jessica-mitchell]

Great! thanks for adding this note @elide-b , I have a couple of suggestions for improvements :)

[jessica-mitchell]

Thanks @elide-b lgtm

[jessica-mitchell]

Merging with 1 reviewer as its documentation related