Add @blocknote/xl-multi-column to docs

[virgile-dev]

Feature Request

Is your feature request related to a problem or unsupported use case? Please describe.
Currently the only way to layout information in columns are tables

Describe the solution you'd like
I'd like to add the multi-column package to our BlockNote implementation so that users can have multi-column blocks.

[virgile-dev]

@AntoLC as we're thinking of doing a blocknote upgrade could we look into adding this as well?
@YousefED did you make it work for export in pdf, docx and odt?

[YousefED]

We did not add export support for multi-column yet unfortunately, but I think we can prioritize it this month