Update how to guides (#840)
* 📝 clarify how to download and upload files
* 🖍 finish section on create/manage a repo
* 🖍 add sign-in section to repo manage guide
* ✨ update inference api section
* 🖍 apply omar review
* 🖍 format links to functions
* 🖍 more review
* 🖍 fix toctree
