DOC: add example for sparse.density and sparse.coo

[Moisan]

• closes #xxxx (Replace xxxx with the GitHub issue number)
• Tests added and passed if fixing a bug or adding a new feature
• All code checks passed.
• Added type annotations to new arguments/methods/functions.
• Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

[MarcoGorelli]

thanks for working on this - does fill_value do anything here?

I'd suggest either:

• writing an example where fill_value has an effect
• just keeping the example above

[Moisan]

to_coo() requires the fill_value to be zero, but when using floats, it is np.nan by default:

>>> df = pd.DataFrame({"A": pd.arrays.SparseArray([0, 1.234, 0, 8.32])})
>>> df.sparse.to_coo()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/thierrymoisan/dev/pandas/pandas/core/arrays/sparse/accessor.py", line 344, in to_coo
    raise ValueError("fill value must be 0 when converting to COO matrix")
ValueError: fill value must be 0 when converting to COO matrix

So I believe fill_value does have an effect in this example.

[MarcoGorelli]

thanks for explaining - to be honest I'd leave this second example out, I'm not sure what it adds (either that, or add a sentence before the example explaining what it does)

[MarcoGorelli]

Nice, thanks @Moisan !