Missing documentation for `slabs` in Biome configuration

[xanderstuff]

This page on the wiki: Biome Configuration does not mention the slabs option, while it is used in the default pack to smooth single-block y-level changes.

[xanderstuff]

I would have also linked the biome file in the default pack I was looking at that uses this slabs option, but I can't find where the default pack is located in the github repos