Document mirrors in SIMBAD and VizieR

There are no mention of the mirrors in the narrative documentation.

```python
from astropy.config import get_config_dir
get_config_dir()
```

Locate the narrative documentation for the SIMBAD and VizieR services and review the get_config_dir() example in the issue. Document the available mirrors and how users can find or configure them, then verify that both service sections clearly mention the mirrors.

Written by the indexing model from the issue text.