Streamline and document nbdev tool usage outside nbdev repos

[seeM]

It'd be helpful if we extended config management to a flow that starts with defaults, overrides with user settings, then project settings, then notebook front matter:

• Streamline default settings #747
• User-level settings #755
• Support nbdev_install_hooks in non-nbdev repos #777
• tutorial: git friendly jupyter #798
• Document nbdev_clean, nbdev_export, nbdev_test usage outside nbdev repos #800
• Make nbdev_export work outside nbdev repos #799

[seeM]

Closing this. We can track it in the smaller issues instead.