DOP-6049 adds new directives called multi-column and column

[caesarbell]

Ticket

DOP-6049

Notes

Adds two new directives (multi-column, and column).

README updates

• • This PR introduces changes that should be reflected in the README.md and/or HACKING.md, and I have made those updates.
• • This PR does not introduce changes that should be reflected in the README.md and/or HACKING.md

[seungpark]

would like to see unit tests to see what the output in AST is

[seungpark]

change to multi column component

[seungpark]

also, would add a note (or option) to note how each column would be presented in small vs big screen. the design shows each column being its own block

[caesarbell]

@seungpark I can change the directive name to multi column component.

[seungpark]

i meant in the help line! im not sure if this is limited to two columns

[caesarbell]

@seungpark gotcha, that makes sense. Right now, from the design, it is. I wanted to leave some flexibility for the writers in case they wanted to use it in another place.