Improve the widgets initialization examples

[m1r0]

The widgets initialization examples are a bit misleading and some users have problems with the autoloading (htmlburger/carbon-fields#532).

[brianpurkiss]

I've run into this issue but it's for blocks instead of widgets.

"Fatal error: Uncaught Error: Class 'Carbon_Fields\Block' not found"

Copy/pasting the blocks code from the documentation page results in this error for me: https://docs.carbonfields.net/#/containers/gutenberg-blocks

Thanks to the above linked thread I should be able to fix this, but I agree, some documentation improvements would be nice.

(thanks for the awesome plugin!)

[brianpurkiss]

I wonder if I found the issue, still digging.

From here: https://carbonfields.net/docs/carbon-fields-plugin-quickstart/

I followed the Without Composer instructions, which means I don't have the autoloader portion used. Maybe my "Fatal error: Uncaught Error: Class 'Carbon_Fields\Block' not found" error is because I didn't use Composer?