Add docstrings for default coordinate frame attributes

### Description

Presently, for example, [the docs](https://docs.astropy.org/en/stable/api/astropy.coordinates.AltAz.html#astropy.coordinates.AltAz) don't list `alt` or `az` as attributes on the `AltAz` object, making them difficult to discover for new users.

A docs fix for this would make `alt` and `az` appear in the "Attributes Summary" section of the `AltAz` page.

Start from the astropy.coordinates.AltAz API documentation page and trace its Attributes Summary to the coordinate-frame attribute definitions. Check how existing attributes are exposed in the generated docs, then document alt and az so they appear in that summary. Verify the rendered AltAz page lists both attributes.

Written by the indexing model from the issue text.