feat(docs): Added Advanced Guides section and module creation page

[nmunnich]

Added an advanced guides section. Also added module creation page so that there would be an item in the section already. Expect additional pages to be based off of this one in other commits.

I am not entirely happy with the modules page as-is, though I think a good part of that is because I'm imagining it being used together with a zmk-modules repo, which doesn't exist yet. The page would need updating when the time comes. It is mostly a cherry-picked version of the corresponding Zephyr page currently, but the far reduced length and focus only on the most relevant bits still makes it worthwhile to have imo.