Move AutoClasses under Main Classes
This PR proposes moving `Auto Classes` under the `Main Classes` section instead. As pointed out by @patrickvonplaten, the docs look a little strange because the `Auto Classes` doc isn't bold like the rest of the section and the header name doesn't exactly fit. Since bolded titles mean it is expandable we can't put `Auto Classes` in bold to align it with the rest of the section. `Auto Classes` also includes other things besides models (config, tokenizer, processor, etc.), so it doesn't exactly fit under `Models`.
_The documentation is not available anymore as the PR was closed or merged._