doc: introduce fact injection workflow

[alejandrosame]

Introduce a workflow to extract documentation fragments from the nixpkgs tree. This way, we can encourage the autogeneration of documentation for sections that desync easily, such as the existing Python interpreters for a given nixpkgs version.

This workflow is introduced in a manual fashion. The contributor willing to autogenerate some fragment has to provide with a fact.target indicating the file where to look for a line <!-- FACT ${fact.id} -->, which will be substituted with the contents of fact.drv before calling nixos-render-docs.

From this point, we can work on further automation/convenience. For example, nixos-render-docs could be extended to accept an include block facts that looks for autogenerated fragments, which will potentially give better feedback to contributors.

Future work

• Add a (proper) generator for python-interpreter-table.

I have a working function already, but needs code and comment refactoring. It could be included as another commit

• Add testing framework to the current workflow.
• Make a roadmap to extend nixos-render-docs with this workflow.

[fricklerhandwerk]

I think we can close this, since the Python table generation is but one sample for how to do it, and we can abstract over it once there's need.