Improve example inner-blocks-dcd824

[juanmaguitar]

This example, in its current state, doesn't show well the behavior enabled by the InnerBlocks component for a block.
Improve this example to:

• Cover some more of the features explained at https://developer.wordpress.org/block-editor/how-to-guides/block-tutorial/nested-blocks-inner-blocks/ like limiting the blocks that can be inserted
• Include some styling to clarify the boundaries of the block containing other blocks