DOC: improve cross-links in docs between .expanding and .cum*
xref #12648 
- cross-link doc-strings for .cum\* to the .expanding().\* (for sum,prod,max,min, which are only supported).
- show an example (in computation.rst wher eexpanding is shown? or basics.rst (where cum\* are shown)) how these are related (w.r.t. NaN filling).

```
In [7]: s = Series([1,2,np.nan,3,np.nan,4])

In [8]: s.cumsum()
Out[8]: 
0     1.0
1     3.0
2     NaN
3     6.0
4     NaN
5    10.0
dtype: float64

In [9]: s.expanding().sum()
Out[9]: 
0     1.0
1     3.0
2     3.0
3     6.0
4     6.0
5    10.0
dtype: float64

In [10]: s.cumsum().ffill()
Out[10]: 
0     1.0
1     3.0
2     3.0
3     6.0
4     6.0
5    10.0
dtype: float64
```
