docs: add move-column-to-workspace focus parameter example

[martinetd]

I was just opening a discussion when I found about it when looking at how hard it'd be to implement, and noticed it was already done recently but no doc was added in #1332 !

Not sure this is the best place to add this, but it's better than nothing -- happy to let you add it somewhere else and close it instead, will likely be faster than iterating on details.

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This feature was added a while ago but nothing in the doc mentions it anywhere. Add a basic usage example.

Fixes: #1332

[martinetd]

BTW is there any way to discover all valid parameters short of reading src/input/mod.rs ?
niri msg action help could perhaps list parameter names somehow?

(but I'll leave that as an exercise for the reader...)