docs: readme refresh

[jamestrew]

Tweaking the readme based on some feedback & common areas on confusion I've been seeing in the issues.

Main changes:

• Added lua table for default options people can copy/paste into their config and tweak for hopefully better discoverability of options.
• Added example keymapping for opening the file browser in the current buffer path
• Reordered sections to bring mappings closer to the usage section

One #210 and #222 are completed, we'll probably want to update the demo gif/mp4s. I can also work on that when the time comes.