Docs: Recommend using a RHEL image

The [current docs](https://github.com/ansible/ansible-builder/blob/68707471bf690a10a362b21ab5eabd4732124e10/docs/index.rst?plain=1#L58) state

```
Choosing a base image
---------------------

You can use any base image you choose.
```

However in practice this is not true, as issues indicate (#636, #553).

This should change to show that RHEL images are essentially required.