Add examples of nested column creation to Fine Data Manipulation document

Now that #150 has merged, the use cases captured by its unit tests should also be introduced in the tutorial documentation, in this document in particular:

https://nested-pandas.readthedocs.io/en/latest/tutorials/data_manipulation.html

Demonstrate examples of the form
```python
nf2 = nf.eval("lc.calibrated_mag = lc.mag + zp")
```
and
```python
nf2 = nf.eval("a + packed.c.median()"
```

Open the Fine Data Manipulation tutorial at the linked ReadTheDocs page and review the use cases covered by #150's unit tests. Add documentation examples for creating nested columns and evaluating an expression over a packed nested column; the tutorial should demonstrate both forms shown in the issue.

Written by the indexing model from the issue text.