docs: restructure docs & document fallback option

[nekowinston]

Documents the new fallback key & cleans up the docs a bit.
I went with the emoji-style headers as popularized by folke/lazy, and pulled up the Lazy.vim example to the top.

I'm not sure we should feature examples like the

	set_dark_mode = function()
		vim.api.nvim_set_option_value('background', 'dark', {})
		vim.cmd('colorscheme gruvbox')
	end,   

in the README, when the vim.cmd shouldn't be required; maybe an example syncing lualine or similar would be more useful.

[f-person]

great updates, lgtm! <3