Add render.enable and render.output options

[MattSturgeon]

Allow downstream users to use the render module, without adding packages to perSystem.packages.

Motivation

In nixvim, we already have a MDBook based documentation site, similar to the flake.parts website.

We'd like to add some pages documenting flake-parts modules, so the render module would be useful to avoid re-implement things like evalWith ourselves.

However we don't need/want to add the generated-docs packages to our flake outputs. Instead, we'd iterate over perSystem.render.inputs ourselves, perhaps adding relevant derivations as passthru attrs on our main docs package.