Add "Guides" section to owlkettle docs
As we chatted about earlier, the nimibook docs should include a `Guides` section.

The intent should be to provide guides for various ways of using owlkettle. This shall be subdivided into further subsections if that ever grows.

The guides should be included for now:
- centralize state across your application by defining ref-types that you pass around

In that case, consider adding links to the [owlkettle-crud example](https://github.com/can-lehmann/owlkettle-crud/tree/main) (and maybe pull the owlkettle-CRUD example into the owlkettle repo?).

Future additions may include:
- How to cross-compile to windows
- How to deploy as a flatpak
- How to deploy as an AppImage
- How to deploy for X-Distro (deb, rpm, AUR)
- Suggestions for multithreading