Documentation needed for Columns Selection DSL

[Jolanrensen]

Aside from merging #372 we also need to update the columns selection page on the website.
Need to look into getting the DSL grammar parts copied over neatly as well.

[Jolanrensen]

We also probably need some more aside from the grammar:

• Big explanation for 3/4 function
  • (We could also take their grammar if we'd want to)
• Small mention of the other functions (probably grouped) with small explanation (more than just a comment in the examples)

Also probably

• An explanation about the grammar (can probably be generated from kdoc)