replace mkdocs with zensical

With the [recent events around the mkdocs universe](https://squidfunk.github.io/mkdocs-material/blog/2026/02/18/mkdocs-2.0/) it seems migrating away from it is the more sensible option for this project. [zensical](https://zensical.org/) seems to already be in a good shape for us, so let's migrate to it 