Add example of nested content with columns / tabs

[jsakas]

Hey there!

I've been learning gutenberg dev over the last few days and I feel there is a strong need for repeatable nested content such as columns or tabs.

I haven't found a great example of a barebones implementation of this but I think it would be really beneficial to this repo and other developers learning this framework. In ACF land the repeater field is one of the most widely used and I sense as we start shifting over to gutenberg blocks we'll have similar needs of creating repeatable content.

I also raised a similar question on Stack Overflow: https://stackoverflow.com/questions/61925625/developing-block-with-multiple-child-blocks-such-as-columns-or-tabs