Training & fine-tuning quickstart (#5034)
* add initial fine-tuning guide
* split code blocks to smaller segments
* fix up trianer section of fine-tune doc
* a few last typos
* Update usage -> task summary link
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
