C API: Publish docs

[roberth]

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

API docs should be available with each release.

Describe the solution you'd like

Either

• publish them directly to nix.dev
• distribute along with the regular manual, optionally
  • add a package.nix parameter that controls the inclusion (false by default)
  • add a the output of the api docs derivation to the doc output
  • link it from the menu
  • add a flake output that builds Nix with api docs enabled
  • publish that instead of the regular doc output

Describe alternatives you've considered

Additional context

Priorities

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