Split config doc into 'basic' and 'advanced'

I think we can probably take 10% of these and put them into a 'basic' config page, for stuff normal people might want to change. [Terminal encoding](https://beets.readthedocs.io/en/stable/reference/config.html#terminal-encoding) etc could be in an advanced/expert page.

https://beets.readthedocs.io/en/stable/reference/config.html

Or does readthedocs support some kind of collapse /hide functionality where we can have them both on one?