BUG: indexing a `QTable` with a `Quantity` column doesn't work as expected

```python
import astropy.units as u
from astropy.table import QTable

t = QTable({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0]]
```
fails with
```python-traceback
Traceback (most recent call last):
File "/Users/clm/dev/astropy-project/coordinated/astropy/t.py", line 6, in
t.loc[t["a"][0]]
~~~~~^^^^^^^^^^^
File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/table/index.py", line 888, in __getitem__
rows = self._get_rows(item)
^^^^^^^^^^^^^^^^^^^^
File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/table/index.py", line 867, in _get_rows
for key in item:
File "/Users/clm/dev/astropy-project/coordinated/astropy/astropy/units/quantity.py", line 1283, in __iter__
raise TypeError(
TypeError: 'Quantity' object with a scalar value is not iterable
```

In contrast, the following works
```python
import astropy.units as u
from astropy.table import QTable

t = QTable({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0].value] # passing `.value` instead of the full quantity
```

Using a `Table` instead of a `QTable` also works as expected
```python
import astropy.units as u
from astropy.table import Table

t = Table({"a": u.Quantity([0], unit="m")})
t.add_index(["a"])
t.loc[t["a"][0]]
```

I'm not 100% sure the current behaviour is actually buggy: this is a matter of intent.
This was first discussed in #15826 where @taldcroft found that multi-indexing wouldn't work seamlessly for `Quantity` indices, as he assumed.

This report is adapted from my comment https://github.com/astropy/astropy/issues/15826#issuecomment-1942541936_

Start by reproducing the QTable example and read astropy/table/index.py around _get_rows (line 867) and __getitem__ (line 888). Review the discussion in issue #15826 to determine the intended behavior for Quantity-valued indexes, then add or update tests so the chosen behavior is explicit and the example is handled consistently.

Written by the indexing model from the issue text.