Add auto model selection

[ntrogh]

Add section to Language Models doc about auto model selection.

Fixes #8744

[isidorn]

Overall looks good. Made minor comments.
Also assigning to @nhu-do in case she has additional feedback. ANd @lramos15 as optional reviewer.

I will start working on the blog that we can release side-by-side.

[isidorn]

Minor comments

[isidorn]

Also consider if there is a FAQ entry at the end of that LM docs page that could be about Auto mode.
Though we can update that ad-hoc based on user feedback

[nhu-do]

Minor comments, thanks for updating the docs!