Allow individual modules to publish documentation

[srid]

This is like #75 - but covers arbitrary documentation - like a tutorial/howto series - but organized in Diataxis framework. So for example, haskell-flake can publish its own tutorials and howtos and they ultimately render nicely in https://flake.parts/ ...

[roberth]

In #627 I added some (stone age) logic to generate the navigation menu for the options docs (reference docs). The same could be done for tutorials, guides and explanations.
Perhaps the inputs could provide directories containing

In render.inputs.<name> we could have an option docs as nullOr path, referring to a directory.
This directory would then contain

• tutorials.json: mapping from title to file
• tutorials/: directory containing those markdown files
• guides.json ... same idea ...
• ...
• intro.md: for the intro in the reference docs

It could then be copied to the right places and we can generate the menus with the json.

EDIT: added intro.md