[Article] Update blockquote behavior

[GemN]

To match the markdown guidelines, a blockquote must be able to embed other markdown components.
This work out of the box with correctly formatted markdown.

> #### The quarterly results look great!
>
> - Revenue was off the chart.
> - Profits were higher than ever.
>
>  *Everything* is going according to **plan**.

give:

The quarterly results look great!

• Revenue was off the chart.
• Profits were higher than ever.

Everything is going according to plan.

• blockquote component add a child paragraph
• blockquote child elements must have a reduced vertical padding
• only show the block menu for the deepest nested child
• allow only a set of blocks and inline

Block menu:

• Paragraph
• Blockquote
• Code
• Heading

Inline menu:

• Bold
• Italic
• Link
• Code