Templates(select) - improve documentation around initial selections

The docs aren't clear about how to go about setting an initial selection for the different selects. It is the same pattern, and the examples use it (setting a selected prop on an option), but it would be helpful to explain this in the docs.

Also, currently you can still pass selected to the templates because the props tables extend SelectProps, but this is currently nonfunctional to a user since state is managed internally. My thought is we can exclude the prop from the extension, and potentially add an initialSelection prop as a more direct way of setting an initial selection (and an alternative to the option prop method that currently exists).

Jira Issue: PF-34

Localiza la documentación de la plantilla, sus ejemplos de select y las tablas de props que extienden SelectProps; empieza comparando cómo se describe el patrón existente de la opción seleccionada en los distintos selects. Se considera terminado cuando el comportamiento de la selección inicial esté documentado con claridad y el tratamiento en la API de selected frente a un posible prop initialSelection esté resuelto y reflejado de forma coherente.