docs: migrate to feel-co/ndg

[NotAShelf]

This PR migrates our entire documentation framework to https://github.com/feel-co/ndg, the tool that we've created to document Nix/OS related projects. While less mature, ndg is a much more flexible utility that we can tweak and improve based on our needs rather than relying on a nixpkgs-constrained utility with the worst error handling I've ever seen.

The current changes rely on an indev version of ndg, which I plan to release shortly. There are a few leftover bugs that need to be fixed before this can be merged, so draft until those two issues are fixed. Until then we can focus on wording and work on making existing pages more approachable by the users.

[github-actions[bot]]

:rocket: Live preview deployed from 26c4a7e3c33e739d474ddaf52aa4c5f3d11922ba

View it here:

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Triggered by: Soliprem

HEAD at: notashelf/push-ysqzsqxwlsml

Reruns: 1668