Use/write an interface definition language for docs and codegen

[roberth]

Is your feature request related to a problem? Please describe.

• We currently rely on a built Nix to provide the inputs for the manual. Rebuilds slow down documentation work, and make it far less accessible to contributors, as it requires a degree of familiarity with the tooling and/or unbearable iteration cycle times.

• This architecture restricts the tooling we can use for documentation.

Describe the solution you'd like

Use an existing interface definition language that has good documentation support, or write a simple one based on JSON.

Describe alternatives you've considered

• A build mode that only compiles a small set of files; just enough to pull out the docs.

  • Build nix itself somewhat incrementally, optionally #8543

• Lean into Nix as a language processing tool. It makes for a nicer end user experience, and perhaps more coherent tooling (despite being somewhat custom). This may be more of an addition than an alternative actually.

  • Described in comment

Additional context

Priorities

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